To keep the malaria parasite in check, multiple antimalarial drugs are commonly blended into a single pill. Such artemisinin-based combination therapies are an effective way of thwarting the evolution of drug resistance, but they also provide a golden opportunity for counterfeiters, as tests designed to identify sham monotherapies don't always work for drug combos.

One common field test, for instance, involves dissolving ground-up combination pills with lab reagents; if the pill contains artemisinin, the solution turns yellow, and a deeper golden hue equals more artemisinin. But at least two antimalarial drugs on the market are already yellow, so the test might change color even if the pill contains no artemisinin, warns Harparkash Kaur, a chemist at the University of London.

The problem can be overcome. One solution is to separate the compounds before performing the assay. Michael Green, a chemist at the US Centers for Disease Control and Prevention in Atlanta, has developed such a test (Trop. Med. Intl. Health 6, 980–982, 2001), but he admits that this necessitates an extra step.

Another workaround is a lab technique called thin-layer chromatography. Global Pharma Health Fund, a charity funded by the pharma giant Merck, sells a $6,800 'minilab' that contains a chromatography test in addition to Green's color test and other anticounterfeit tools. As Nature Medicine went to press, the charity had shipped 347 minilabs to 70 countries.

But the minilab chromatography test requires ultraviolet lamps, which can burn out under strenuous field conditions, Kaur points out. So, last year she and Jean-Robert Ioset, of the Drugs for Neglected Diseases initiative in Geneva, developed an easy-to-read assay that reveals how much artemisinin is found in a pill through colored spots that can be viewed without the aid of ultraviolet light (PLoS One 4, e7270, 2009).

Even as these new tests come on board, however, the extent of the counterfeit problem remains unclear. That's why Kaur, Green and others—with backing from the Bill & Melinda Gates Foundation—are now working to determine the prevalence of counterfeit antimalarials in Africa. Knowing this will help researchers pinpoint the causes of emerging drug resistance, Kaur says.

And getting a grip on the problem is an important first step toward solving it, adds Patrick Lukulay, who heads a project aimed at rooting out counterfeit drugs in developing countries at US Pharmacopeia, a Maryland-based nonprofit. “Knowledge,” he says, “will lead to action on the part of the authorities.”