Claudia Martínez, the head of research at the Access to Medicine Foundation, an Amsterdam-based non-profit group, described the finding as a major public health concern.
Estimates published last year by the UN Office on Drugs and Crime put the human cost of falsified and substandard medicines at up to 500,000 deaths a year in sub-Saharan Africa.
A World Health Organization (WHO) spokesperson said antibiotics and antimalarial products were the most falsified medicines in Africa.
Substandard or falsified antibiotics can contain incorrect dosages or the wrong active ingredients, leading to ineffective treatments and survival of resistant strains. The WHO said such products were likely to be fuelling an increase in antimicrobial resistance.
Malawi was found to have the highest proportion of substandard and falsified medicines, according to the study.
Martínez said several factors contributed to the problem, which left patients without access to essential medicines.
Pharma supply chains in many low- and middle-income countries are often complex, inefficient and fragmented; the region relies heavily on a limited number of suppliers for essential medicines, and many countries face significant challenges in procuring products in time and effectively policing the quality of products in the market.
A previous study by the WHO found that an estimated one in 10 medical products in developing countries were substandard or falsified, with 42% of reports of substandard and falsified medicines coming from Africa, 21% from the Americas and 21% from Europe.
A Guest Editorial