functional abdominal pain ( fap ) is one of the most common diseases , and large percentages of children suffer from it . some studies showed a prevalence of 9% - 15% ( 1 ) . according to the rome iii criteria , the functional abdominal pain must include all of the following criteria : persistent or episodic abdominal pain insufficient criteria for other functional gastrointestinal disorders no evidence of an anatomic , inflammatory , metabolic or malignancy disorders these criteria should be present at least once per week for at least 2 months prior to diagnosis ( 2 ) . existence of fap in childhood can increase the risk of irritable bowel syndrome ( ibs ) ( 3 ) , predictors of psychiatric disorders and other psychosomatic symptoms in adulthood ( 4 ) . the prevalence of celiac disease ( cd ] in children with fap was about two times the normal population ( 5 ) . current recommendations for treatment of fap include : reassuring parents , high - fiber diet , antidepressants and peppermint oil ( 6 ) . it was indicated that probiotics are more effective than placebo in the treatment of fap ( 7 ) . they also prevent bacteria from proliferating in the small intestine and are also immune system modulators ( 8) . most of the studies have shown that the use of these agents is safe , effective and well - tolerated ( 9 ) . some studies have demonstrated that the number of intestinal lactobacillus and bifidobacterium in children with fap and ibs are decreased ( 12 ) . therefore , the use of these microorganisms can be effective in the treatment of fap . the purpose of this study was to determine the efficacy of lactobacillus reuteri ( biogaia ) for treating chronic functional abdominal pain in children . it was conducted on 80 children aged 4 to 16 years , who visited the gastroenterology clinic of ayatollah mousavi hospital in zanjan from 2012 - 2013 ( around 14 months ) . all 4 - 16 years old children with at least 1 attack of abdominal pain per week during the past 2 months were evaluated for functional abdominal pain ( fap ) based on rome iii criteria ( 2 ) . all parents were given enough information about the study and consent to participation in the project obtained . inclusion criterion included : - all children aged 4 to 16 years with functional abdominal pain ( fap ) according to rome iii diagnostic criteria exclusion criteria included : - having an abdominal pain with known organic cause   - having a history of abdominal and gastrointestinal surgery - having ftt or weight loss more than 5% of body weight - having any abnormal paraclinical finding including complete blood count , urinalysis , stool examination for occult blood , biochemistry , abdominal ultrasound , liver function tests , serum amylase and lipase - having a history of drug use in the past 3 months including antidepressants or laxatives   - having any kind of chronic illness - having a history of abdomen blunt trauma the patients were divided into two groups based on the randomized allocation ( computer registration ) . the first group ( case group ) was given probiotic from lactobacillus family and reuteri strain ( biogaia brand manufactured by sweden health care company ) 5 drops per day orally equivalent to 108 cfu ( colony - forming units ) for 4 weeks . the physicians and the patients were unaware of the contents of the medications prescribed ( double blind study ) . patients were followed up for 4 weeks after cessation of treatment . in follow up period frequency and severity of pain , other symptoms and drug side effects were carefully recorded . demographic data and pain characteristics ( frequency , location , presence or absence of associated symptoms , need to sedative for pain relief ) were recorded . pain intensity was assessed according to the wong - baker faces scale system ( figure 1 ) ( 13 ) . the scale includes six face images showing the effect of the pain , ranging from relax face ( no pain ) in the left side and intense face ( most severe pain ) in the right side . follow up the researcher assessed intensity pain scores , frequency of pain , and ultimately response to treatment . finally the data were analyzed with spss software and using descriptive statistics , the frequency distribution chart , chi - square test and relative risk was calculated . in all stages of the analysis , p - value equal or less than 0.05 ( p - value  0.05 ) was considered statistically significant . during a 14-month study , 270 children with abdominal pain were referred to the pediatrics gastroenterology clinic . of these , only 80 children fulfilled the criteria of functional abdominal pain ( rome iii ) , which were divided into two groups . forty children were in the case group ( biogaia recipient group ) half of whom were boys . other children ( 40 cases ) were in the control group ( placebo recipient group ) , 52.5% of these were males and 47.5% females . the mean and standard deviation of age in the case and control group were 6.26  2.10 and 6.26  2.61 years , respectively . there was no statistically significant difference in terms of mean and standard deviation of weight ( table 1 ) ( p = 0.99 ) . at baseline 97.5% of children in case group and 90% of the control group did not have any associated symptoms . fever and anorexia were seen in 2.5% and 7.5% of children in control group , respectively . in this sense , both groups were consistent ( p = 0.16 ) , 95% of patients in the case group and 97.5% in the control group had periumbilical pain , and the remaining had hypogastric pain . intensity of abdominal pain in the case group according to scoring system of wong - baker faces scale was about 30% more , 57.7% even more and 12.5% whole . in the control group , there was no significant difference between the groups ( table 2 ) . at the end of the second month , 40% in case group and 65% in control group were no hurt . zero score is used for no hurt and 6 for whole . applying this scoring system , average score obtained in the control group at baseline was 4.05  0.71 , at the end of first month 2.08  1.56 and at the of second month 2.25  1.46 . in the case group corresponding scores were 3.83  0.63 , 2.50  1.45 and 2.53  1.43 , respectively . according to these results , there was significant difference between the groups at baseline and first month after treatment ( p = 0.0001 ) , while there was no significant difference at the end of second and first month in both groups ( in case group p = 0.317 and control group p = 0.227 ) . at first month , 50% of children in case group and 65% in control group had no pain and at second month 47.5% in case group and 52.5% in control group did not report any pain . in this respect average pain episodes per week in the control group was 1.33  0.47 at baseline , 0.40  0.59 at first month and 0.530.59 at second month . in the case group it was 1.45  0.55 at baseline , 0.68  0.76 at first month and 0.70  0.75 at second month . here also , there was a significant change in both groups compared to the first month ( p = 0.0001 ) , but the change was not significant in the second month compared with the first month ( in case group p = 0.317 and control group p = 0.059 ) . this study investigated the effects of lactobacillus reuteri and placebo in reducing severity and frequency of pain in 80 children aged 4 to 16 years with chronic functional abdominal pain . functional gastrointestinal disorders ( fgid ) are one of the most common childhood disorders , pathophysiology of which is still not well defined ( 14 ) . some of the studies have shown that the number of lactobacillus is significantly reduced in the intestine of these patients ( 15 , 16 ) . thus , replacement of these microorganisms was an acceptable hypothesis for treatment of functional abdominal pain . in our study both cases and control groups in terms of age , sex , weight , number of pain episodes per week , location of pain and associated symptoms were consistent . however , there was no significant difference between the groups in the intensity and frequency of pain episodes . after one month of cessation of drugs , no significant change was observed compared to the first month . our study revealed that use of probiotics in treating fap had no preference to placebo . in literature reports regarding the effect of lactobacillus reuteri in the treatment of fap niv et al . as in our study , found that the effect of lactobacillus reuteri was similar to placebo in reducing abdominal pain and no more than that ( 17 ) . unlike our study , claudio romano et al reorted that lactobacillus reuteri reduces the severity of pain in children but , similar to our results does not reduce the frequency of pain ( 18 ) . the dose of probiotic was chosen based on previous published studies ( 19 ) , its manufacturer s recommendations ( 109 cfu ) and duration of treatment as mentioned in the literature ( 20 ) . however , the optimal dose and duration of drug therapy in the treatment of fap is not clearly identified yet . in this study , we treated the samples randomly divided into two groups , so that the groups were matched for age and sex . other limitation of the study was not having access to patients over 13 years old . most of them were referred to the adults gastroenterology clinic and could not be included in the study . so , average age of the patients in our study was considered lower than that in other studies . to achieve appropriate management of fap our results showed that , although lactobacillus reuteri may be effective in reducing severity and frequency of pain , its effect is not greater than placebo . again our study proved that the use of placebo in the treatment of functional gastrointestinal disorders can be helpful , which emphasizes the fact that main causes of the disorder are mental and emotional problems , and also environmental stress , whereas the organic causes are less involved . the major contribution to the treatment of fap is to ensure the child and parents , so stress could be reduced .