medication overuse headache ( moh ) is a chronic headache disorder due to overuse of analgesic medicines , taken to treat a primary headache , and remitted by drug discontinuation . moh has long been considered as a  biobehavioural  disorder , in which the biological predisposition to develop a chronic headache interacts with an individual behaviour prone to develop a dependence disorder [ 2 , 3 ] . the molecular basis of moh are largely unknown , though it has been hypothesized that this subtype of headache could be mediated by cognitive impulsivity , and shares some pathophysiological mechanisms with drug addiction , including dysfunction of the frontostriatal system and central pain sensitization induced by drug overuse . based on this hypothesis , there seems to be a role in moh for bdnf ( brain - derived neurotrophic factor ) , because of its role in central pain sensitization and its relationship with substance - related disorders [ 5 , 6 ] . bdnf is a member of  neurotrophin  family , a unique group of polypeptide growth factors that influence differentiation and survival of neurons in the developing nervous system . bdnf is involved in long - term potentiation ( ltp ) , and therefore in learning and memory mechanisms . also , a pathogenic role for bdnf has been shown in pain transduction and sensitization , in epilepsy , and in neuropsychiatric disorders . bdnf gene , mapped to chromosome 11p , encodes for pro - bdnf that is subsequently cleaved in its mature form . recently , it has been observed that a common 196 g  >  a single - nucleotide polymorphism ( snp ) of bdnf gene , resulting in a valine at residue 66 to methionine ( val66met ) substitution in pro - bdnf protein , exhibits a functional relevance in that it seems to reduce bdnf activity by altering neurotrophin trafficking . the val66met variant affects anatomy , cognition and behaviour in adults   and leads to memory impairment   and susceptibility to obsessive - compulsive , eating , substance abuse and mood disorders [ 6 , 810 ] . the rationale behind our study derives from two findings , which have been consistently reported in previous researches : ( 1 ) overuse of analgesic drugs is a defining feature of moh and , for this reason , moh has been related to behavioural syndromes associated with substance abuse [ 13 ] , ( 2 ) there is evidence that proneness to use / abuse substances is associated with the allele a of bdnf polymorphism .therefore , we tested the frequency of the val66met genotype in a sample of moh patients , and compared clinical features between individuals homozygous for g allele ( g / g ) and carriers of the allele a ( non - g / g ) . aim of this study is to confirm our hypothesis that allele a of the examined bdnf polymorphism interfere with moh , seeking for clinical and psychiatric differences between the two groups , mainly in the monthly drug consumption , that is both the clinical hallmark of disease , and the feature closest to abuse disorder . we would like to emphasize that , although such studies are necessary to get a deeper understanding of the association between moh and bdnf polymorphism , the present study did not intend to ascertain whether the bdnf polymorphism is a risk factor for the development of moh , but only an aggravating factor for subjects that have already developed the disorder . unrelated consecutive moh patients were enrolled from two italian university headache clinics : the inpatient headache - unit of irccs  c. mondino  institute of neurology foundation ( pavia ) , and headache outpatient department of university of rome  la sapienza  , among those admitted with a diagnosis of probable moh , to undergo withdrawal from their overused medications . although revised criteria were formulated for moh , patients enrolment was started before their statement , thus original ichd - ii version ( i.e. in the presence of headache improvement within 2  months after analgesics withdrawal ) , that is more conservative , was adopted to recruit all patients ; a retrospective analysis will also confirm that all enrolled patients fulfilled the revised criteria . socio - demographic and clinical data were obtained by anamnesis ; consumption of analgesic drugs per month ( from here , monthly drug number ) , days with headache per month ( from here , monthly headache days ) and days with drug consumption per month ( from here , monthly drug days ) were self reported by patients through a headache diary , received at the time of first visit . inclusion criteria were : ( a ) patient written informed consent , ( b ) absence of other major medical conditions , ( c ) absence of psychiatric disorders that could sustain a chronic pain condition ( psychotic and/or somatoform disorders ) , ( d ) to have accurately completed the last 3-month pre - withdrawal diary , ( e ) ability to complete the psycho - diagnostic assessment , ( f ) diagnostic confirmation of moh after withdrawal . presence of categorical psychiatric diagnosis was investigated through four modules of the structured clinical interview for dsm - iv axis i disorders ( scid - i ) , clinician version : mood episodes , mood disorders , anxiety disorders , and eating disorders . moreover , a dimensional assessment was made with the beck depression inventory ( bdi ) , a self - reported questionnaire to measure the presence and severity of depressive symptoms . with the patients written informed consent , 10  cc of peripheral blood were collected , total genomic dna purified , and genotyping for the bdnf val66met - bdnf variant was performed essentially as reported . all data obtained in the three phases of the study ( socio - demographic and clinical recordings , psychiatric assessment , and genetic determinations ) were merged in a all - comprehensive database by an independent data - manager , which opened the blind and performed the sample size analysis on a preliminary group , to indicate the minimal number of patients needed in the study , and statistical analyses on definitive data . on the base of initial 36 patients , a sample size analysis was performed to obtain the required number of subjects to fulfil a desired power of 80% with significance level at 5% . because our primary endpoint was to detect a difference in number of analgesics consumed per month by patients between the two groups ( see introduction ) , we divided g / g from non - g / g subjects and calculated the monthly mean of drug consumption : 21 non - g / g subjects consumed meanly 45.57 doses versus 32.93 doses of 15 g / g subjects ( t  =  1.887 , p  =  0.07 , cohen s d  =  0.65 ) . sample size resulted to be 87 subjects , respectively , 51 for non - g / g and 36 for g / g group . descriptive statistics to compare bdnf genotypes were performed using parametric or , when appropriate , non - parametric tests . a stepwise multiple linear regression analysis was employed to identify significant predictors of the monthly drug number in the entire sample . in addition , to well - know indexes of null hypothesis significance testing , effect size measures were also reported to recognize the value of the degree of association among variables , particularly between bdnf polymorphism group and monthly drug number [ 17 , 18 ] . effect sizes were calculated with the phi (  ) coefficient [ or , when appropriate , cramr s v ( c ) ] for  test , with cohen s d for student s t test , with partial eta squared \documentclass[12pt]{minimal } 
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				 \begin{document}$$ ( \eta_{\text{p}}^{2 } ) $ $ \end{document } for the analysis of variance ( anova ) , and with cohen s f for multiple regression . unrelated consecutive moh patients were enrolled from two italian university headache clinics : the inpatient headache - unit of irccs  c. mondino  institute of neurology foundation ( pavia ) , and headache outpatient department of university of rome  la sapienza  , among those admitted with a diagnosis of probable moh , to undergo withdrawal from their overused medications . although revised criteria were formulated for moh , patients enrolment was started before their statement , thus original ichd - ii version ( i.e. in the presence of headache improvement within 2  months after analgesics withdrawal ) , that is more conservative , was adopted to recruit all patients ; a retrospective analysis will also confirm that all enrolled patients fulfilled the revised criteria . socio - demographic and clinical data were obtained by anamnesis ; consumption of analgesic drugs per month ( from here , monthly drug number ) , days with headache per month ( from here , monthly headache days ) and days with drug consumption per month ( from here , monthly drug days ) were self reported by patients through a headache diary , received at the time of first visit . inclusion criteria were : ( a ) patient written informed consent , ( b ) absence of other major medical conditions , ( c ) absence of psychiatric disorders that could sustain a chronic pain condition ( psychotic and/or somatoform disorders ) , ( d ) to have accurately completed the last 3-month pre - withdrawal diary , ( e ) ability to complete the psycho - diagnostic assessment , ( f ) diagnostic confirmation of moh after withdrawal . presence of categorical psychiatric diagnosis was investigated through four modules of the structured clinical interview for dsm - iv axis i disorders ( scid - i ) , clinician version : mood episodes , mood disorders , anxiety disorders , and eating disorders . moreover , a dimensional assessment was made with the beck depression inventory ( bdi ) , a self - reported questionnaire to measure the presence and severity of depressive symptoms . with the patients written informed consent , 10  cc of peripheral blood were collected , total genomic dna purified , and genotyping for the bdnf val66met - bdnf variant was performed essentially as reported . all data obtained in the three phases of the study ( socio - demographic and clinical recordings , psychiatric assessment , and genetic determinations ) were merged in a all - comprehensive database by an independent data - manager , which opened the blind and performed the sample size analysis on a preliminary group , to indicate the minimal number of patients needed in the study , and statistical analyses on definitive data . on the base of initial 36 patients , a sample size analysis was performed to obtain the required number of subjects to fulfil a desired power of 80% with significance level at 5% . because our primary endpoint was to detect a difference in number of analgesics consumed per month by patients between the two groups ( see introduction ) , we divided g / g from non - g / g subjects and calculated the monthly mean of drug consumption : 21 non - g / g subjects consumed meanly 45.57 doses versus 32.93 doses of 15 g / g subjects ( t  =  1.887 , p  =  0.07 , cohen s d  =  0.65 ) . sample size resulted to be 87 subjects , respectively , 51 for non - g / g and 36 for g / g group . descriptive statistics to compare bdnf genotypes were performed using parametric or , when appropriate , non - parametric tests . a stepwise multiple linear regression analysis was employed to identify significant predictors of the monthly drug number in the entire sample . in addition , to well - know indexes of null hypothesis significance testing , effect size measures were also reported to recognize the value of the degree of association among variables , particularly between bdnf polymorphism group and monthly drug number [ 17 , 18 ] . effect sizes were calculated with the phi (  ) coefficient [ or , when appropriate , cramr s v ( c ) ] for  test , with cohen s d for student s t test , with partial eta squared \documentclass[12pt]{minimal } 
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				 \begin{document}$$ ( \eta_{\text{p}}^{2 } ) $ $ \end{document } for the analysis of variance ( anova ) , and with cohen s f for multiple regression . a hundred and seven moh patients ( 84 women and 23 men ; age , mean    sd : 46.741    11.36  years ) were enrolled in the study . depending on the type of overused drug , patients were divided into four groups : 29 patients ( 27.10% ) consumed exclusively triptans , 18 ( 16.82% ) exclusively non - steroidal antinflammatory drugs ( nsaids ) ; 27 ( 25.23% ) exclusively combination drugs ( combination ) , and 33 ( 30.84% ) associated different types of drugs ( association ) . none of the patients consumed opioid drugs ; all types of combination drugs consumed by our patients resulted to contain caffeine . by genotyping entire moh population , we identified seven cases of harboured a / a genotype ( 6.54% ) , 53 were g / a ( 49.53% ) , and 47 were g / g ( 43.93% ) . samples were in hardy - weinberg equilibrium (   =  2.460 , df  =  1 , p  =  0.12 for the null hypothesis ) . a preliminary analysis was performed to investigate differences in monthly drug numbers between a / a ( mean    sd 54.86    19.05 ) , a / g ( 49.06    27.19 ) and g / g ( 33.83    14.04 ) group . due to small sample size in a / a group whitney u test , and the monte carlo method , a bootstrapping technique with a resampling process that produces nonparametric significance estimates . the three groups differed statistically ( among them ) in number of monthly used drug [ mean rank : a / a , 76.21 ; a / g , 61.43 ; g / g , 42.31 ; h  =  13.317 , df  =  2 , p  <  0.0001 ( 99% confidence interval < 0.00010.001 ) ] . pairwise comparisons ( significance threshold adjustment with bonferroni s correction , p  =  0.0167 ) showed higher values in a / a [ mean rank : a / a , 43.00 ; g / g , 25.19 ; u  =  56.000 , z  =  2.794 , p2-tailed  =  0.004 ( 99% ci 0.0030.006 ) ] and a / g [ mean rank : a / g , 58.82 ; g / g , 41.12 ; u  =  804.500 , z  =  3.046 , p2-tailed  =  0.002 ( 99% ci 0.0010.003 ) ] when compared with g / g patients , but no difference between a / a and a / g [ mean rank : a / a , 37.21 ; a / g , 29.61 ; u  =  138.500 , z  =  1.082 , p2-tailed  =  0.29 ( 99% ci 0.280.30 ) ] . till now obtained results confirmed our decision to consider together carriers of allele a in the non - g / g group : ( 1 ) above - mentioned explorative analysis showed that a / a and a / g groups were not different in terms of monthly drug consumption , ( 2 ) as expected , a little size of a / a subjects was found , and it makes difficult to perform complex , multivariate statistical analysis with an acceptable degree of reproducibility . as mentioned , to perform further statistical analyses , the population was divided into two groups : g / g genotype ( 47 subjects , 43.93% ) , homozygotes for valine ( allele g ) , and non - g / g genotype ( 60 subjects , 56.07% ) , carriers of metionine 66 ( allele a ) , either in heterozygosity or in homozygosity . clinical characteristics and descriptive statistics for the two groups are illustrated in detail in table  1 . non - g / g showed a higher consumption of monthly drug number ( non - g / g vs. g / g , mean    sd : 49.73    26.31 vs. 33.83    14.04 ; t  =  4.011 , p  <  0.0001 , d  =  0.76 ) and longer moh duration ( 6.43    7.78 vs. 3.09    3.66 ; t  =  2.944 , p  =  0.004 , d  =  0.56 ) when compared to g / g patients . there were no significant differences between g / g and non - g / g patient groups in terms of other clinical and socio - demographic features . moreover , no significant associations were found among the two groups and current occurrence of psychiatric disorders.table  1table shows descriptive socio - demographic , headache , and psychiatric data for the entire study samplenon - g / g ( n  =  60 ; 56.07%)g / g ( n  =  47 ; 43.93%)statisticsgender  women47 ( 43.93%)37 ( 34.58%)  =  0.002p  =  0.96  =  0.005  men13 ( 12.15%)10 ( 9.35%)age ( years)47.57    11.4744.94    11.15 t  =  1.192p  =  0.24d  =  0.23education ( years)12.34    4.4012.59    4.09 t  =  0.262p  =  0.79d  =  0.06employment  yes38 ( 35.51%)32 ( 29.91%)  =  0.263p  =  0.61  =  0.050  no22 ( 20.56%)15 ( 14.02% )  age of primary headache onset ( years)16.20    7.4215.32    7.14 t  =  0.620p  =  0.54d  =  0.12  primary headache duration ( years)32.07    11.9630.43    14.28 t  =  0.647p  =  0.52d  =  0.13  age of moh onset ( years)41.53    11.8242.66    12.64 t  =  0.475p  =  0.64d  =  0.09  moh duration ( years)6.43    7.783.09    3.66 t  =  2.944p  =  0.004d  =  0.56  monthly headache days24.82    6.0725.89    5.70 t  =  0.935p  =  0.35d  =  0.18  monthly drug number49.73    26.3133.83    14.04 t  =  4.011p  <  0.0001d  =  0.76  monthly drug days23.63    6.1422.17    6.46 t  =  1.195p  =  0.24d  =  0.23drug type  triptans18 ( 16.82%)11 ( 10.28%)  =  0.707p  =  0.87c  =  0.081  nsaids10 ( 9.35%)8 ( 7.48% )  association17 ( 15.89%)16 ( 14.95% )  combination15 ( 14.02%)12 ( 11.21%)current psychiatric diagnosis  present45 ( 42.06%)34 ( 31.78%)  =  0.096p  =  0.76  =  0.030  absent15 ( 14.02%)13 ( 12.15%)current mood diagnosis  present27 ( 25.23%)22 ( 20.56%)  =  0.035p  =  0.85  =  0.018  absent33 ( 30.84%)25 ( 23.36%)current anxiety diagnosis  present32 ( 29.91%)24 ( 22.43%)  =  0.054p  =  0.82  =  0.023  absent28 ( 26.17%)23 ( 21.50%)current eating diagnosis  present6 ( 5.61%)2 ( 1.87%)  =  1.257p  =  0.26*  =  0.108  absent54 ( 50.47%)45 ( 42.06%)current double psychiatric diagnosis  present18 ( 16.82%)15 ( 14.02%)  =  0.045p  =  0.83  =  0.021  absent42 ( 39.25%)32 ( 29.91% )  bdi ( score)13.15    8.5312.09    8.73 t  =  0.634p  =  0.53d  =  0.12continuous data are expressed as mean    sd , whereas categorical data are frequencies and percentages of total subjects . differences between non - g / g and g / g patients , with bivariate statistics results , are also illustrated . bold entries represent the statistical significant differencesmoh medication overuse headache , bdi beck depression inventory , monthly drug days number of days per month in which patient assumed at least one dose of drug to treat headache , drug type triptans ( exclusively one , or more , types of triptan ) , nsaids ( exclusively one , or more , type of non steroidal antinflammatory drugs ) , association ( use of different types of drug in association ) ; combination ( drugs of combination , with more active ingredients ) *  fisher s exact test , two - tailed : p  =  0.46 table shows descriptive socio - demographic , headache , and psychiatric data for the entire study sample continuous data are expressed as mean    sd , whereas categorical data are frequencies and percentages of total subjects . differences between non - g / g and g / g patients , with bivariate statistics results , are also illustrated . bold entries represent the statistical significant differences moh medication overuse headache , bdi beck depression inventory , monthly drug days number of days per month in which patient assumed at least one dose of drug to treat headache , drug type triptans ( exclusively one , or more , types of triptan ) , nsaids ( exclusively one , or more , type of non steroidal antinflammatory drugs ) , association ( use of different types of drug in association ) ; combination ( drugs of combination , with more active ingredients ) *  fisher s exact test , two - tailed : p  =  0.46 a two - way analysis of covariance ( ancova ) was employed to investigate the group effect ( two between - subject factors :  genotype  , non - g / g vs. g / g ;  drug type  , triptans vs. nsaids vs. association of drugs vs. drugs of combination ) on monthly drug number ( dependent variable ) ; monthly headache days and monthly drug days were used as covariate in this analysis to reduce the error variance . the ancova model was highly significant ( f9,97  =  7.149 , p  <  0.0001 ) and explained more than one - third of variance ( adjusted r  =  0.343 ) . the  drug type effect was not significant ( f3,97  =  0.383 , p  =  0.77 , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \eta_{\text{p}}^{2 } = 0.0 1 1 $ $ \end{document } ) whereas the  genotype  effect was statistically considerable ( f1,97  =  12.487 , p  =  0.0006 , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \eta_{\text{p}}^{2 } = 0 . 1 1 4 $ $ \end{document } ) ; the interaction effect of  genotype drug type  was also significant ( f3,97  =  2.776 , p  =  0.04 , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \eta_{\text{p}}^{2 } = 0.0 7 9 $ $ \end{document } ) ( fig .  1 ) . among the covariate , monthly drug days was highly significant ( f1,97  =  16.918 , p  <  0.0001 , \documentclass[12pt]{minimal } 
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				 \begin{document}$$ \eta_{\text{p}}^{2 } = 0 . 1 4 9 $ $ \end{document } ) but there was no effect of monthly headache days ( f1,97  =  0.043 , p  =  0.84,\documentclass[12pt]{minimal } 
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				 \begin{document}$$ \eta_{\text{p}}^{2 } = 0.000 $ $ \end{document } ) . pairwise comparisons [ fisher s least square difference ( lsd ) ; significance threshold adjustment for multiple comparison test with bonferroni s correction ( 0.05/8 ) : p  =  0.00625 ] showed no genotype difference in patients who consumed triptans ( p  =  0.74 ) and nsaids ( p  =  0.16 ) ; on the contrary , when compared with g / g , non - gg patients had a statistically higher consumption of drug both among those that used association of drugs ( p  =  0.009 ; this value , however , did not reach the adjusted threshold of significance ) and among consumers of drugs of combination ( p  <  0.0001).fig . 1interaction effect of  genotype      drug type  on monthly drug number . markers ( black circles and squares ) indicate mean , vertical bars denote 0.95 confidence intervals interaction effect of  genotype      drug type  on monthly drug number . markers ( black circles and squares ) indicate mean , vertical bars denote 0.95 confidence intervals as showed in table  2 , in the multiple regression analysis , the val66met bdnf polymorphism ( beta  =  0.33 , t  =  3.669 , p  <  0.0001 , f  =  0.134 ) and the bdi score ( beta  =  0.19 , t  =  2.090 , p  <  0.04 , f  =  0.037 ) emerged as significant , independent predictors of the monthly drug consumption in moh patients . no further variables , considered as possible predictors , entered the regression model ( see table  2).table  2results of stepwise multiple regression analysis : bdnf polymorphism ( g / g genotype ) and bdi score ( depressive symptoms ) are the predictors of monthly drug number in moh patientsvariablebetatpbdnf gene0.333.669<0.0001bdi0.192.090<0.04moh duration0.121.2420.22drug type0.080.9170.36gender0.080.9080.37employment0.070.7540.45current psychiatric diagnosis0.050.5320.60age of headache onset0.020.1670.87age0.010.1230.90significant predictors are in boldstepwise model summary : adjusted r  =  0.137 , f2,104  =  9.424 , p  <  0.0001 , f  =  0.181stepwise model change statistics : first step , variable entered  =  bdnf gene , r change  =  0.118 , f1,105 to enter / to remove  =  14.030 , p  <  0.0001 , f  =  0.134 ; second step , variable entered  =  bdi , r change  =  0.036 , f1,104 to enter / to remove  =  4.368 , p  <  0.04 , f  =  0.037moh medication overuse headache , 
 bdnf brain - derived neurotrophic factor , bdi beck depression inventory results of stepwise multiple regression analysis : bdnf polymorphism ( g / g genotype ) and bdi score ( depressive symptoms ) are the predictors of monthly drug number in moh patients significant predictors are in bold stepwise model summary : adjusted r  =  0.137 , f2,104  =  9.424 , p <  0.0001 , f  =  0.181 stepwise model change statistics : first step , variable entered  =  bdnf gene , r change  =  0.118 , f1,105 to enter / to remove  =  14.030 , p  <  0.0001 , f  =  0.134 ; second step , variable entered  =  bdi , r change  =  0.036 , f1,104 to enter / to remove  =  4.368 , p  <  0.04 , f  =  0.037 moh medication overuse headache , 
 bdnf brain - derived neurotrophic factor , bdi beck depression inventory consistent with our working hypothesis , monthly drug consumption in moh patients resulted to be influenced by the examined bdnf polymorphism ( fig . 2 ) , both if univariate , and both if multiple regression analyses ( to control the association for such possible confounding factors ) are performed . the relationship between monthly drug consumption and val66met polymorphism was also confirmed by effect size calculation ( medium effect at t test , small to medium effect at regression analysis ) . the fact that a unique bdnf snp could account for these observed effects is noteworthy , because drug consumption is a multifactorial variable , surely influenced by many other genetic and environmental factors.fig . g group , the black circles show a / a patients , the grey ones with black ring a / g patients . vertical bars denote 0.95 confidence intervals monthly drug number in g / g and non - g / g patients . g group , the black circles show a / a patients , the grey ones with black ring a / g patients . vertical bars denote 0.95 confidence intervals moh can be considered as a clinical condition at the boundaries between drug addiction and chronic headache . the hypothetical involvement of bdnf in the pathophysiology of moh was suggested by : ( 1 ) the association between the val66met polymorphism and drug abuse disorders , ( 2 ) the role of bdnf in modulating central and peripheral pain . in this study group , non - g / g patients showed a more severe clinical picture , because they had used a higher number of analgesic drugs in the previous 3  months , and had longer disease duration . showing for the first time a role in moh , our findings confirm that somehow the examined bdnf polymorphism is involved , as an independent factor , in pathophysiology of this headache condition . although it might sound too approximate , considering the association between met66 pro - bdnf and a reduced bdnf activity , it is tempting to speculate that in moh a lower bdnf activity is a worsening factor . if moh is regarded as a drug abuse disorder , our data would agree with previous reports of an association between the val66met polymorphism and substance addition . the proneness to abuse ( i.e. to have an impulse dyscontrol ) , previously reported in non - g / g subjects , would be expressed in moh patients by higher drug consumption . in light of this hypothesis , in fact , although the defining factor in moh is the number of days with drug intake , the variability of this value theatrically ranges from 10 to 30 ; on the contrary , a hypothetic upper limit for variability of monthly drug consumption does not exist and patients could be distributed on a wider range that better reflects their proneness to abuse ( and/or headache severity ) . previous studies have attempted to evaluate bdnf role in headache disorders . in particular , it has been demonstrated an increased level of bdnf in cerebrospinal fluid ( csf ) of patients with a chronic daily headache ( with or without drug overuse ) , in comparison to healthy controls . furthermore , it has been shown a reduced level of platelet bdnf in the interictal phase of episodic migraineurs and cluster headache patients , in comparison to healthy controls . more recently , a bdnf elevation in csf was demonstrated both in chronic migraine and in fibromyalgic patients . these three studies were much different from our research for two reasons : the use of diverse classification criteria in the first study ; in general , the different study designs , with the lack of relations among bdnf levels , clinical markers of disease severity , and the patients bdnf genotype . however , these findings showed the involvement in chronic pain pathophysiology of bdnf up regulation , and might support indirectly our hypothesis that in moh patients a bdnf hypo - activity , related to val66met polymorphism , does not influence pain severity , but impulsive behaviour . to the best of our knowledge , pending specific studies on this topic , only speculations could be made about the chance that the higher drug intake is a consequence of putative bdnf - mediated higher pain sensitivity , or not . however , the pain modulation is linked to bdnf release and up - regulation [ 23 , 24 ] , whereas val66met polymorphism is related with a reduction of bdnf activity . these observations , though indirectly , might lend support the idea that bdnf influence on moh is played in terms of impulsivity modulation , as in other drug abuse disorder , and not pain hypersensitivity . moreover , a recent paper analysed influence of val66met polymorphism on episodic migraine . examined polymorphism resulted neither associated to development of migraine ( comparing polymorphism frequency in a population of episodic migraineurs with a healthy control group ) , nor associated with differences in terms of clinical characteristics ( within the only migraine group ) . consistent with these data , because val66met polymorphism seems to be unable to influence development and clinical features of migraine , we suppose that observed worsen effect of polymorphism on moh is likely driven by the a allele related proneness to abuse , and , in turn , that the monthly drug consumption in moh patients is not necessarily the expression of headache severity . as reported above , we have also recorded the type of drug consumed by each patient and they were segregated in four groups : triptans , nsaids , association of different type of drugs and drugs of combination . it is noteworthy that differences between g / g and non - g / g emerged only in the fourth group ( drugs of combination , all containing caffeine ) , and a trend was also found for the association of drugs . if one consider that non - g / g subjects are supposed to have a bdnf hypoactivity , and that bdnf is a kind of biological reward for presynaptic neurons , our observations could mean that the lack of reward induces non - g / g subjects in the continue search of different type of drugs , or in caffeine seeking ( for its psychotropic effect ) . none of the patients reported to use drugs containing opioids ; therefore , we can not confirm our hypothesis with this class of drugs . severity of depressive symptoms ( as measured by bdi score ) was found as another independent predictor of drug consumption in moh patients . the role of depression in headaches has been previously reported and discussed elsewhere [ 26 , 27 ] . although a relation between depression and val66met polymorphism was frequently found , in this study the g / g and non - g / g groups neither differ statistically for the severity of depressive symptoms , nor were they associated significantly with a psychiatric disorder . maybe these associations are covered in our population by already described relationship between moh and psychiatric comorbidity that could not be further increasable by examined polymorphism . moreover , observed lack of association suggests that this bdnf polymorphism and depression might have an effect on moh monthly drug consumption through different and unrelated mechanisms . in summary , our results showed an influence of bdnf polymorphism val66met on drug consumption in moh patients . the polymorphism is likely not related to severer forms of headache , but with a bdnf hypoactivity , in turn related to substance abuse behaviour , consistent with observed higher caffeine seeking . therefore , aggravating effect of examined polymorphism in moh patients could be driven by its proneness to develop an impulsivity dyscontrol , the cornerstone of any substance abuse disorder . in other words , we suppose that moh is not only a very severe headache , but also a complicated bio - behavioural disorder , not dissimilar to a kind of addictive disorder , preferentially developed by some migraineurs , aggravated by conditions unrelated to previous headache clinical features ; however , related to behavioural impulsivity dyscontrol , as other substance abuse disorders . moreover , we would like to highlight that analysis of snps within functionally related genes appears as a good tool in headache research to identify mechanisms potentially involved in disease pathogenesis .