many drug patents have recently expired or are scheduled to expire in the near future . in response , many drug manufacturers have expanded their generic drug portfolio , which requires them to conduct clinical trials that demonstrate that their generic equivalents perform similarly to the innovator drug product . regulations introduced by the united states food and drug administration ( fda ) and the european medicines agency ( ema ) over the last thirty - five years have strengthened measures to ensure the bioequivalency of drug products , which may be simultaneously manufactured by multiple drug makers [ 3 - 5 ] . bioequivalence and bioavailability testing standards have also emerged following recognition that bioinequivalence and variations in the bioavailability of drug products can result in therapeutic failure and/or toxicity [ 6 - 8 ] . in the united states , the successful approval of new and abbreviated new drug applications requires regulatory approval by the fda . recent studies have suggested that this process takes nearly a decade to complete the required series of pre - clinical studies and clinical trials . drug development is an expensive process that is marked by a high - failure rate . for these reasons , early stage bioequivalence and pharmacokinetic studies are essential in determining the fate of new drug products . in this study , we sought to systematically assess the current trends of ongoing and recently completed bioequivalence and bioavailability trials that have been registered within a national clinical trials registry . this study provides insight regarding the characteristics of current bioequivalence and bioavailability trials and may also provide assistance in prioritizing future areas of research . ] we identified bioequivalence and bioavailability trials registered in clinicaltrials.gov using the key words  bioequivalence  and  bioavailability  . briefly , clinicaltrials.gov is a publicly - available registry of clinical research studies that is maintained by the u.s . national institutes of health . as of mid-2013 there were nearly 150,000 studies registered in clinicaltrials.gov , with study sites in 185 countries ( http://www.clinicaltrials.gov ) . our search was restricted to identify studies registered between 01 october 2007 and 31 december 2012 to coincide with the enactment of a federal law in 2007 that mandated the registration of all phase 2 - 4 interventional trials involving drugs , biological agents , and medical devices . we excluded all observational trials ( n=34 ) as well as trials that were  suspended ( n=7 ) ,  terminated  ( n=33 ) , or  withdrawn  ( n=26 ) . the remaining trial registry entries were systematically examined and the following data elements were extracted : a unique trial identifier , study title , recruitment status , phase ( 0 - 4 ) , study design , blinding status , interventional assignment to trial arms , primary endpoint classification , primary purpose of the trial , age group and gender eligibility criteria , and anticipated enrollment size . descriptive statistics were used to characterize the bioequivalence and bioavailability trials identified in the clinicaltrials.gov registry . comparisons between ongoing trials and those that have been completed were performed using the -test or fisher s exact test . all statistical analyses were undertaken in stata 11.2 ( statacorp lp , college station , tx , usa ) . we identified bioequivalence and bioavailability trials registered in clinicaltrials.gov using the key words  bioequivalence  and  bioavailability  . briefly , clinicaltrials.gov is a publicly - available registry of clinical research studies that is maintained by the u.s . national institutes of health . as of mid-2013 there were nearly 150,000 studies registered in clinicaltrials.gov , with study sites in 185 countries ( http://www.clinicaltrials.gov ) . our search was restricted to identify studies registered between 01 october 2007 and 31 december 2012 to coincide with the enactment of a federal law in 2007 that mandated the registration of all phase 2 - 4 interventional trials involving drugs , biological agents , and medical devices . we excluded all observational trials ( n=34 ) as well as trials that were  suspended ( n=7 ) ,  terminated  ( n=33 ) , or  withdrawn  ( n=26 ) . the remaining trial registry entries were systematically examined and the following data elements were extracted : a unique trial identifier , study title , recruitment status , phase ( 0 - 4 ) , study design , blinding status , interventional assignment to trial arms , primary endpoint classification , primary purpose of the trial , age group and gender eligibility criteria , and anticipated enrollment size . descriptive statistics were used to characterize the bioequivalence and bioavailability trials identified in the clinicaltrials.gov registry . comparisons between ongoing trials and those that have been completed were performed using the -test or fisher s exact test . all statistical analyses were undertaken in stata 11.2 ( statacorp lp , college station , tx , usa ) . from october 2007 through december 2012 there were 2,388 interventional bioequivalence and bioavailability trials registered in clinicaltrials.gov . of these , 227 ( 10% ) trials are actively recruiting participants , 15 ( 1% ) are recruiting by invitation only , 87 ( 4% ) are engaged in data analysis , and 2,059 ( 86% ) have been completed . ongoing bioequivalence and bioavailability trials are more likely to be in later phase clinical trials , as reflected by a decrease in the proportion of phase 0 , 1 , and 1/2 trials from 75% among completed studies to 36% of ongoing studies ( p<0.001 ) . ongoing trials are also more likely to be double - blinded ( 27% vs. 12% ; p<0.001 ) and have larger sample sizes ( p<0.001 ) . similarly , ongoing trials are more likely to feature parallel group assignment and less likely to be crossover trials ( p<0.001 for both ) . there has also been an increase in the proportion of trials that primarily involved research on treatments from 42% to 55% ( p<0.001 ) . the proportion of trials that exclusively recruited male participants declined from 20% to 9% ( p<0.001 ) and the number of trials that enrolled children increased from 3% to 17% ( p<0.001 ) . more than half of the bioequivalence and bioavailability trials registered in clinicaltrials.gov were conducted internationally ( 58% ) . among ongoing trials , the majority of ongoing trials are recruiting participants in north america ( 52% ) and europe ( 26% ) ; however , east asia ( 7% ) , the middle east ( 4% ) , and south america ( 4% ) are also involved in several ongoing bioequivalence and bioavailability trials . from october 2007 through december 2012 there were 2,388 interventional bioequivalence and bioavailability trials registered in clinicaltrials.gov . of these , 227 ( 10% ) trials are actively recruiting participants , 15 ( 1% ) are recruiting by invitation only , 87 ( 4% ) are engaged in data analysis , and 2,059 ( 86% ) have been completed . ongoing bioequivalence and bioavailability trials are more likely to be in later phase clinical trials , as reflected by a decrease in the proportion of phase 0 , 1 , and 1/2 trials from 75% among completed studies to 36% of ongoing studies ( p<0.001 ) . ongoing trials are also more likely to be double - blinded ( 27% vs. 12% ; p<0.001 ) and have larger sample sizes ( p<0.001 ) . similarly , ongoing trials are more likely to feature parallel group assignment and less likely to be crossover trials ( p<0.001 for both ) . there has also been an increase in the proportion of trials that primarily involved research on treatments from 42% to 55% ( p<0.001 ) . the proportion of trials that exclusively recruited male participants declined from 20% to 9% ( p<0.001 ) and the number of trials that enrolled children increased from 3% to 17% ( p<0.001 ) . more than half of the bioequivalence and bioavailability trials registered in clinicaltrials.gov were conducted internationally ( 58% ) . among ongoing trials , the majority of ongoing trials are recruiting participants in north america ( 52% ) and europe ( 26% ) ; however , east asia ( 7% ) , the middle east ( 4% ) , and south america ( 4% ) are also involved in several ongoing bioequivalence and bioavailability trials . this study reveals that bioequivalence and bioavailability trials are part of a global clinical research enterprise . when compared to completed trials , ongoing trials are in later phases of clinical development , recruiting larger numbers of participants , and more likely to recruit women and children . these data suggest that bioequivalence and bioavailability studies are undergoing a transformation as drug makers seek to characterize the safety and efficacy of drug products in more rigorous trials that closely resemble their anticipated patient population . as the costs of healthcare and drug development have risen , there is a mounting incentive for improving our understanding of existing treatments while also enabling breakthrough discoveries . recently , the united kingdom has attempted to strategically align their clinical research funding with their public health priorities . although similar measures have not been enacted in the united states , it behooves policy makers to consider the vital role that bioequivalence and bioavailability studies play in bringing new and generic drug products to the public . as noted here , the quality of bioequivalence and bioavailability studies has improved rapidly , even over the last five years , and the horizon is bright . however , as the national debate on healthcare reform and research priorities unfolds we may need to re - evaluate our approach to bioequivalence and bioavailability trials to ensure that safe and efficacious medicines swiftly reach healthcare providers and their patients .