You watch what you eat and you work out, but you still gain weight. The reason might be in your medicine cabinet.

Ginny Berlin's grandchildren take her breath away, literally. Diagnosed with emphysema, Berlin began taking the prescription steroid Prednisone to open up her lungs.

"For the last year I've pretty much been on a dose every single day," said Berlin.

But within a few months, she noticed puffiness in her face, arms and chest. Berlin gained twenty pounds from Prednisone.

"I went to a size four to a size eight, so I had to buy new clothes," said Berlin.

"Corticoid steroids, the most common one people know is prednisone," said Harminder Sikand, pharmacist.

Steroids treat a lot of different ailments.

"They're used for every disease known to man. They're used for allergies, treatment of cancer, they're used for appetite stimulants because they cause weight gain," said Sikand.

Steroids increase appetite and cause water retention. Children can gain weight from medicines like Benadryl, a common over the counter anti-histamine.

"It's used for itching, allergy symptoms, used for getting your kids to sleep at night," said Sikand.

Researchers believe anti-histamines lower metabolism, which makes your body store more fat.

Another widely used medication is antidepressants. Medications such as Paxil and Prozac can expand your waistline. They're called serotonin re-uptake inhibitors, and they increase appetite. But there's another possible explanation.

"You might be losing weight from depression, and we're treating depression, so you're feeling better and now, you're eating more," said Sikand.

But some diabetics eat less and still gain weight on anti-diabetic pills.

"These medications get you to produce more insulin and so they think they help with the production of fat," said Sikand.

Blood pressure medicine, with beta-blockers, such as Inderal, Lopressor - also commonly known as Metroperol or Propernol - also seem to slow the rate calories are burned.

"They are many other alternatives to use," said Sikand.

Just knowing which pills may plump could actually tip the scale in your favor.

"There are preventive things you need to do if you know if this is going to cause you to gain weight - low calorie diet, increase your exercise," said Sikand.

Berlin says it's bothersome, but it's not going to prevent her from taking her medication.

Doctors say the benefit of taking medicine almost always outweighs the risk of gaining weight. But if you're worried about it, ask before you start taking the medication. In some cases, alternatives are available.