The commonly used diabetes drug metformin stinks, literally, and this may explain why many patients stop taking it, U.S. doctors reported on Monday.The drug smells like fish or dirty socks to some people and this could account for the well-known side effects of the drug, which can make people nauseated, they said.

But the problem could be solved by coating the pills so they do not smell or release the odor into the stomach, where it can be burped up, they wrote in a letter to the Annals of Internal Medicine.

"We wonder why this reaction to metformin has not been previously reported," Dr. Allen Pelletier of the Medical College of Georgia and colleagues wrote in a letter to the journal.

"Patients may report that metformin nauseates them but do not further elaborate or distinguish this as a visceral reaction to the smell of the medication."

They described two cases in detail.

The first had taken brand-name metformin (Glucophage, made by Bristol-Myers Squibb) for several years before being switched to an immediate release, generic version of metformin, which he refused to take.

"He reported that it smelled like 'dead fish' and nauseated him," they wrote. An extended release generic version, coated to make it dissolve more slowly, solved the problem.

A second man refused to ever take metformin again, even coated formulations, they said.

"Our cases show that the distinctive odor of metformin (independent of other, well-known gastrointestinal adverse effects of the medication) causes patients to stop taking the drug," they wrote.

Doctors may simply think patients are having the other side-effects such as diarrhea, nausea and vomiting, flatulence, distention and abdominal pain, but the smell could make patients feel ill, Pelletier and colleagues said.

"Although reaction to the odor of metformin has not been reported in the medical literature, hundreds of postings to message boards on the Internet note the peculiar odor of the drug, which is also well known to pharmacists," they added.

"Trial of a film-coated, extended-release formulation may be a reasonable approach in such cases," they said.

Doctors often struggle to persuade patients to take their diabetes drugs as directed and this could be one easy fix, they said.