En español | Prozac. Insulin. Prednisone. Depakote. Sometimes even Benadryl. These — and dozens of other well-known and widely used prescription and over-the-counter medications — can counter depression, control diabetes, ease arthritis, slow seizures or stop sneezing in allergy season.

Sign up for AARP's Health Newsletter

A prescription drug or over-the-counter medicine could be making you fat. — Photo by Getty Images

They also can make you fat, says George Blackburn, M.D., associate professor of surgery and nutrition at Harvard Medical School, and director of the Center for the Study of Nutrition Medicine at the Beth Israel Deaconess Medical Center in Boston.

Doctors stress that you should never stop taking a drug because you suspect it's making you gain weight. "Don't just stop," says Lillian F. Lien, M.D., medical director of the Duke Inpatient Diabetes Management program. "That's the most dangerous thing to do."

But do watch the scale. If you see a weight gain of "five pounds or more, circle back to the care provider and nip this in the bud," says Blackburn.

Doctors and obesity researchers say weight gain is associated with a few categories or families of drugs, not necessarily some specific brands. And not everyone who takes these medicines puts on pounds. Happily, those who do usually have an alternative medication they can try.

According to Blackburn, the types of drugs associated with weight gain for some patients include:

• Diabetes medicines, including insulin.

• Steroids and hormones, for arthritis and similar conditions, such as prednisone.

• Some over-the-counter allergy medicines like Benadryl (also used as sleep aids for obese patients).

• Some heart and blood pressure medicines, such as both beta-blockers and alpha-blockers.

• Many drugs for depression and mental illnesses, like Elavil, Luvox, Eskalith and Zyprexa.

• Anticonvulsants for epilepsy or other neurologic conditions.

Next: Which blood pressure drugs aren't linked to weight gain? >>