More than one-third of malaria medicines tested in Southeast Asia and sub-Saharan Africa were fake or faulty, according to a new study that warns shoddy drugs could fuel the rise of hardier parasites.

Drugs meant to combat malaria are catnip for counterfeiters because of high profits and scant punishment for fakes, the new study published in the Lancet Infectious Diseases journal says. Many countries suffering from malaria outbreaks lack laboratories equipped to check whether drugs are legitimate.

Weak drugs could jeopardize the strides made over the last decade in stopping malaria as more resistant parasites survive and breed, worsening the problem. As many as 1.2 million people die annually from the disease, which exacts its heaviest toll on children in much of Africa and Southeast Asia.

A team of researchers funded by the U.S. National Institutes of Health pored over published and unpublished studies of thousands of drug samples dating back more than a decade to gauge the extent of the problem. The studies spanned 28 countries including Cambodia, Myanmar, Nigeria and Kenya.