The “fishy” odor is especially noticeable in the immediate release versions.
“Metformin is an excellent drug, but the immediate-release formulation may have an odor to it. The smell is fishy or like the inside of an inner tube, and in a patient’s mind, because it smells like something that has gone bad, they may think the drug isn’t good,” explained one of the letter’s authors, J. Russell May, a clinical professor at the University of Georgia College of Pharmacy at the Medical College of Georgia.
However, May said, “some metformin products on the market are extended-release and the drug is embedded and released slow, over time. These products have much less smell, if any.”
May and his colleagues wrote the letter to the journal to raise awareness of this issue, especially because nausea is a commonly reported side effect of metformin. “Is it nausea from the medication, or is it because it smells bad?” May said.
The drug’s odor may make it seem like it has gone bad, but doctors have reassured patients that it is just something in the formula of the medication, and the drug is still affective at treating diabetes and is not dangerous.
Bristol-Meyers Squib who is one of the major producers of metformin released a statement saying,
Bristol-Myers Squibb is aware that the inherent characteristics of metformin have been associated with a mild odor upon opening of the bottle, so these type of reports are not unexpected. It’s important to note there has been no correlation between an odor and the efficacy of metformin, which has been on the market in the U.S. since 1995.
Patients prescribed metformin should continue on their regular regiment, but should certainly let their doctor know if they are affected by the smell. They may be able to switch to the extended release version or possibly another brand.