LONDON (AP) -- Millions of free malaria drugs are sent to Africa every year by international donors. New research is now providing evidence for what health workers have long suspected: some of the donated medication is being stolen and resold on commercial markets.

During three periods from 2007 to 2010, American and British experts bought malaria medicines randomly from private pharmacies in 11 African cities. Of the 894 samples, they found 58, or 6.5 percent, were supposed to have been donated to government hospitals and clinics.

The study will be published Thursday in the journal Research and Reports in Tropical Medicine and was paid for by the Legatum Institute, a U.S. philanthropic group with no ties to drug makers.

The finding was particularly strong in artemesinin combination drugs, the best available malaria drugs, and those often purchased by international donors. In 2007, they found about 15 percent of such donated drugs had been stolen for resale. This year, it was nearly 30 percent.

The authors acknowledge the sample sizes were small and could exaggerate the problem. Outside experts said donated drugs regularly disappear across corruption-plagued Africa and that the research was credible. There have been no large-scale published studies analyzing the problem.

"The study is important because it clearly documents something that we need to study (the issue) more closely," said Tido von Schoen-Angerer, a director at Medecins Sans Frontieres, which works across Africa.

Von Schoen-Angerer said it is extremely difficult to determine the scale of the problem since drugs are not often followed from their origin to their ultimate destination in Africa.

According to an audit last year by the U.S. President's Malaria Initiative, about $640,000 worth of medicines sent to Angola vanished from airports and the government's medicines warehouse.

"Critical malaria commodities are not reaching their intended beneficiaries and more Angolans may be unnecessary victims," the report said.

"We've heard about this kind of corruption anecdotally for years," said Julian Harris, a health expert at International Policy Network, a London-based think tank. He was not linked to the study. "But the response from funders has been to keep throwing millions of dollars' worth of these medicines into countries, even when there is evidence the drugs aren't reaching the needy."

In the study, Roger Bate, a fellow at American Enterprise Institute in Washington DC, and colleagues focused on the most popular artemesinin combination malaria drug, Coartem, made by Novartis AG. They bought it in private markets in Ghana, Kenya, Tanzania, Uganda, Rwanda and Nigeria. Other drugs from Sanofi Aventis, Cipla and dozens of other companies were also included.

Novartis makes two versions of the drug for Africa: the one to be donated comes in a flat white packet with a blister sheet of pills while the one for commercial markets is sold in an orange and white box.

Bate, an economist who studies health policy, found donated drugs originally meant for Nigeria on sale in Kenya, drugs with "Not for Sale" stamped on them, and drugs packaged in the wrong local language, which suggests they were stolen from aid deliveries.

Novartis declined to comment on the issue.

The donated medicines were first bought by the Global Fund to fight AIDS, Tuberculosis and Malaria and the U.S. President's Malaria Initiative, a joint program led by the U.S. Agency for International Development and the U.S. Centers for Disease Control and Prevention. Both the Global Fund and the President's Malaria Initiative receive funds from international donors, including U.S. taxpayers.

The Global Fund did not respond directly to questions about their drugs were being stolen. But in a recent review, the Fund discovered loopholes in its distribution system and admitted medicines could end up in commercial markets. It has previously suspended grants to Mauritania, Uganda and Zambia when it couldn't track where its money was going.

Some experts dismissed the theft of donated drugs as a major health concern.

Dr. David Sullivan, an associate professor in the Malaria Research Institute at Johns Hopkins Bloomberg School of Public Health, said he did not condone the theft of drugs, but thought the supply of high-quality drugs - like those bought by international donors - to private markets was actually a good thing.

He said about half of Africans go to private health clinics, often flooded with cheap drugs that don't work. "The methods of getting better drugs is not ideal, but from a public health perspective, it's better that effective drugs are available in private clinics."

But Henry Emboho Wanyama, a researcher in Uganda's malaria control department, said stolen drugs can have deadly consequences. "Drugs sent to health units are stolen by medical workers and the malaria patients who go there for treatment are told there are no drugs," he said.

"The patients who cannot afford (to buy) drugs ... end up dying."

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.