Scientists report in a collection of journal articles that up to 41 per cent of specimens failed to meet quality standards in global studies of about 17,000 drug samples.
One of the articles described the discovery of falsified and substandard malaria drugs that caused an estimated 122,350 deaths in African children in 2013.
Other studies identified poor quality antibiotics, which may harm health and increase antimicrobial resistance.
However, new methodologies are being developed to detect problem drugs at the point of purchase and show some promise, scientists said.
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"This problem continues to spread globally, creating an even greater challenge to cooperation among stakeholders, many with limited resources," noted Joel Breman, senior scientist emeritus at the National Institutes of Health's Fogarty International Center.
"The need is urgent for collaboration among those with expertise in policy, science, technology, surveillance, epidemiology and logistics, in order to secure global supply chains," said Breman.
Scientists inspected the quality of about 16,800 samples of anti-malarials, anti-tuberculosis medicines, antibiotics and anti-leishmaniasis drugs and reported from 9 to 41 per cent failed to meet the specifications.
"The pandemic of falsified and substandard medicines is pervasive and underestimated, particularly in low- and middle-income countries where drug regulatory systems are weak or non-existent, as shown by field studies in the supplement," said Jim Herrington, director of the University of North Carolina's Gillings Global Gateway at Chapel Hill.
An urgent and coordinated international response is required to address the pandemic of poor quality drugs, the scientists said.
The research articles are published in The American Journal of Tropical Medicine and Hygiene.