Tests in West Africa have found that a safe drug long used to treat urinary tract infections is also effective against malaria. But the medication has one disadvantage: It turns urine a vivid blue.

“This is something we need to solve, because it could stop people from using it,” said Teun Bousema, a microbiologist at Radboud University Medical Center in the Netherlands and an author of the study, which was published last week in the Lancet Infectious Diseases.

Modern malaria treatments, based on the drug artemisinin, are effective in Africa and save hundreds of thousands of lives each year. But scientists fear the parasites will develop resistance, as they have in Southeast Asia, and are seeking ways to kill them faster using cocktails of several drugs.

Methylene blue, a dye used to stain tissues viewed under a microscope, can be taken by tablet or injection, and is sometimes used to treat urethral infections and a hemoglobin disorder.

But the dye also kills the malaria parasites in the gametocyte stage, the point at which mosquitoes pick it up from human blood and pass it on to new victims.

Most malaria drugs do not target gametocytes, meaning that someone may still spread the disease for a week or more after treatment.

Dutch, U.S. and Malian scientists working in Ouélessébougou, Mali, found that adding methylene blue to a typical treatment regimen cleared gametocytes from the bloodstream within two days.

The 80 study participants, all teenage boys or young men, were warned about the unusual side effect, Bousema said, so there were no surprises. But some were unhappy.

“Most were fascinated, rather than shocked,” he said. “The blue urine did cause stains in the clothing of the mothers of patients, and that was considered a disadvantage.”

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