Tests of 195 different packs of malaria drugs sold in six African cities showed 35 percent of them either did not contain high enough levels of active ingredient or did not dissolve properly.

"Our study shows that efforts to increase access to quality antimalarial drugs in Africa are increasingly important," Dr. Roger Bate of the American Enterprise Institute, who led the study, said in a statement.

"Substandard drugs not only endanger lives today, but also jeopardize future malaria treatment strategies by accelerating parasite resistance."

Substandard antimalarial drugs cause an estimated 200,000 avoidable deaths each year, Bate and colleagues reported in the Public Library of Science journal PLoS ONE.

The researchers said they sent agents to pose as customers at randomly selected pharmacies. They simply asked for antimalarial drugs and the manufacturers were not named in the study.

They said a third of the packs they tested contained only one drug, an artemisinin-based drug. The World Health Organization has asked countries to stop using these so-called monotherapies.

"The high persistence of substandard drugs and clinically inappropriate artemisinin monotherapies in the private sector risks patient safety and, through drug resistance, places the future of malaria treatment at risk globally," the researchers wrote.

Of these one-drug packs, 42 percent failed the quality tests, Bate and colleagues reported.

"The World Trade Organization, which sets the rules of global commerce, should enact rules prohibiting the international trade in artemisinin monotherapies and reducing the tariffs on proper medicines to zero," they wrote.

"These incidents argue strongly for a rule against purchasing locally manufactured medicines, except where those medicines have received regulatory approval from a developed country or the WHO's prequalification scheme," they added.

The WHO estimates that malaria kills 1.3 million people each year, mostly children under age 5.

The disease, transmitted by mosquitoes carrying tiny malaria parasites, has been very hard to fight. The parasites have evolved resistance to the drugs and the mosquitoes have developed resistance to pesticides.

Combinations of drugs work better to kill the parasites and using a cocktail, as well as proper dosing, helps prevent the development of resistance.