Objective To explore the evidence available of poor-quality (counterfeit and substandard) medicines in the literature.

Design Systematic review.

Data sources Databases used were EMBASE, MEDLINE, PubMed and the International Pharmaceutical Abstracts, including articles published till January 2013.

Eligibility criteria Prevalence studies containing original data. WHO definitions (1992) used for counterfeit and substandard medicines.

Study appraisal and synthesis Two reviewers independently scored study methodology against recommendations from the MEDQUARG Checklist. Studies were classified according to the World Bank classification of countries by income.

Data extraction Data extracted: place of study; type of drugs sampled; sample size; percentage of substandard/counterfeit medicines; formulations included; origin of the drugs; chemical analysis and stated issues of counterfeit/substandard medicines.

Results 44 prevalence studies were identified, 15 had good methodological quality. They were conducted in 25 different countries; the majority were in low-income countries (11) and/or lower middle-income countries (10). The median prevalence of substandard/counterfeit medicines was 28.5% (range 11–48%). Only two studies differentiated between substandard and counterfeit medicines. Prevalence data were limited to antimicrobial drugs (all 15 studies). 13 studies involved antimalarials, 6 antibiotics and 2 other medications. The majority of studies (93%) contained samples with inadequate amounts of active ingredients. The prevalence of substandard/counterfeit antimicrobials was significantly higher when purchased from unlicensed outlets (p<0.000; 95% CI 0.21 to 0.32). No individual data about the prevalence in upper middle-income countries and high-income countries were available.

Limitations Studies with strong methodology were few. The majority did not differentiate between substandard and counterfeit medicines. Most studies assessed only a single therapeutic class of antimicrobials.

Conclusions The prevalence of poor-quality antimicrobial medicines is widespread throughout Africa and Asia in lower income countries and lower middle-income countries . The main problem identified was inadequate amounts of the active ingredients.