Researchers in California found that auranofin, an oral therapy for arthritis that has been around since 1985, is highly effective against the parasite Entamoeba histolytica.
Carried in water and food, the parasite is a major but often neglected hazard in poor countries, causing amoebic dysentery and liver abscesses.
Auranofin was found thanks to a hi-tech programme to screen potential drugs for "orphan" diseases.
It was tested on parasites in a lab dish, then in mice with amoebic colitis and on hamsters with amoebic liver cysts, according to the study, published in the journal Nature Medicine.
The experiments suggest auranofin would be 10 times more effective than metronidazole, the current medication for amoebic infection, which means that it could be used in very small doses or even as a one-off tablet.
Better still, the drug has long been recognised as safe.
At recognised dosages, it has few side effects, whereas metronidazole can cause nausea, vomiting, dizziness and headaches.
"This is a drug that you can find in every country," said Anjan Debnath of the University of California at San Francisco in a press release.
"Based on the dosage we're seeing in the lab, this treatment could be sold at about $2.50 per dose, or lower. That cost savings could make a big difference to the people who need it the most."
The discovery was made thanks in part to US federal funding to identify drugs which tackle a neglected, or "orphan," disease.
This definition applies to a disease with fewer than 200,000 cases in the United States or a drug which is effective but whose costs of development and marketing are unlikely to be recovered.
Auranofin works by targeting an enzyme that protects the parasite from oxygen, to which it is highly sensitive, the researchers said.
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