Up to eight percent of malaria drugs approved by the World Health Organization or other regulators do not contain the right dose and may fuel resistance, researchers said Tuesday.
Two studies in published in Research and Reports in Tropical Medicine suggested manufacturing problems, rather than counterfeiting, may be to blame for these substandard drugs in low and middle income countries around the world.
Poorly made or fake medications are a major problem worldwide, particularly as signs emerge of growing resistance to artemisinin, the frontline treatment for malaria, a disease the UN's World Health Organization (WHO) says killed 655,000 people in 2010.
One study sampled 104 artemisinin-based medicines from pharmacies in Ghana and Nigeria that were offered under the Affordable Medicines Facility by the Global Fund to Fight AIDS, designed to boost access to the best treatments.
Eight were found to be significantly under-dosed in artemisinin, or had less than 75 percent of the main component of the drug, the study authors said.
"Why am I pretty sure that these are not fakes?" asked Roger Bate, lead author and resident scholar at the American Enterprise Institute.
"The reason is they have well over 50 percent of the active ingredient but under 75 percent," he said, adding that most counterfeits have very little or no active ingredient in them.
A separate study that spanned 2,652 essential drugs - including medicines for malaria, tuberculosis and bacterial infections - found that the failure rate among WHO-approved products was 6.8 percent.
The samples came from 11 sites in Africa, three Indian cities, as well as Sao Paulo, Moscow, Bangkok, Istanbul and Beijing.
The highest failure rate - 17.65 percent - was seen among Chinese-made products approved by the WHO, said the study.
Researchers said the WHO has launched an investigation.