(CNN) -- A relatively obscure drug used to treat mouth sores has made obese mice thin -- and the mice didn't have to eat less or exercise more, according to a study at the University of Michigan.
Doctors at the university will begin studies in human beings later this year. Hopes are high, even though many drugs that work in mice do not work in humans.
"It is a tour de force and offers a new and potentially exciting opening for developments of new anti-obesity drugs -- something which is badly needed," says Dr. George Bray, chief of the Division of Clinical Obesity and Metabolism at Pennington Biomedical Research Center at Louisiana State University.
In the study at the University of Michigan, researchers gave mice a high-fat diet, and they became obese. They then injected the animals with a drug called amlexanox, which has been on the market for more than 15 years to treat canker sores. The mice lost weight, even though they kept eating the same number of calories. When they were taken off the drug, the mice gained all the weight back.
If amlexanox works in humans -- and that's a big if -- Bray said it could become like the story of Viagra, which was originally developed to treat one condition, chest pain, but serendipitously was later found to treat a different condition, erectile dysfunction.
"It could well be that what works for one disease will turn out to work for another, even though that wasn't the original intent," he said.
The researchers screened several drugs and found that amlexanox, used in an ointment to treat canker sores, changed the action of genes that control metabolism. When injected in mice, the drug worked by increasing metabolism, not by suppressing appetite.
"One of the reasons that diets are so ineffective in producing weight loss for some people is that their bodies adjust to the reduced calories by also reducing their metabolism, so that they are 'defending' their body weight," says Dr. Alan Saltiel, the lead researcher at the University of Michigan. "Amlexanox seems to tweak the metabolic response to excessive calorie storage in mice."
The findings were published Sunday in the journal Nature Medicine.
Clinical trials are expected to begin later this year to test the drug's effectiveness in humans.
In the mouse study, the researchers took pains to make sure that it was the drug, and not something else, that was making the mice lose weight. They used a laser machine to trace the mice to see if perhaps the weight loss was because they moved around more while on the drug.
"They don't move more. The movement is exactly the same. The increased energy expenditure was due to something called thermogenesis, meaning they generate a little bit of heat," Saltiel said.
If the drug helps humans lose weight, researchers would have to make sure it's safe.
"One concern will be whether these drugs, by increasing energy expenditure and body temperature, may have unintended consequences on other systems, such as the heart."
Experts say that's an especially important concern because if the drug is ever used in humans for weight loss, patients would likely need to stay on the drug indefinitely, or else see the weight return.