Antidepressants: Tricyclic medicines can add as many as nine pounds a month; lithium-based mood stabilizers, two and a half pounds. Another class of antidepressants, SSRIs, target the mood-and-appetite-related neurochemical serotonin and may also cause weight gain. If you begin to gain on one of these, look into switching to a bupropion drug; these target neurochemicals that don't increase hunger.
Antipsychotics: Haloperidol and clozapine can have a big effect on metabolism and appetite, adding as many as five pounds a week. Usually people on these drugs are already being closely monitored by a psychiatrist, so if the pounds start to add up, don't hesitate to ask about alternatives such as atypical antipsychotics, which appear to be weight neutral.
Antihistamines, Sleep Aids: Many over-the-counter allergy remedies and sleeping pills contain diphenhydramine, an ingredient that can leave you drowsy during the day and interfere with your sleep patterns at night, reducing the number of calories you're burning.
Blood Pressure Medication: Both alpha- and beta-blockers can cause fatigue, which may add pounds in some patients (the amounts reported vary wildly). If your energy fades, look into ACE inhibitors and calcium channel blockers.
Cancer Therapy: Women with breast cancer are likely to gain weight during chemotherapy. The exact reasons for this are poorly understood, but doctors believe the treatment can slow metabolism.Also, the anti-estrogen drug tamoxifen may increase appetite; Decadron, a steroid used on cancer patients, is another potential culprit. Additionally, chemotherapy often induces early menopause, which can add pounds. Switching drugs isn't an option, so work with your doctors to develop an eating-and-exercise plan.
Diabetes Drugs: Insulin helps process blood sugar by depositing it into cells. Insulin and drugs known as sulfonylureas can bring on bouts of hypoglycemia (low blood sugar), which stimulates appetite. Some patients report gaining up to 11 pounds during the first three to 12 months of treatment. Ask about weight-neutral medications, such as metformin.
Migraine Medicines: Those based on valproic acid can stimulate hunger. These days, doctors are more likely to prescribe Topamax or Imitrex. Neither medicine is associated with weight gain, and both are thought to be safer overall.
Steroids: Oral corticosteroids, commonly used to treat conditions like rheumatoid arthritis and chronic inflammation, add pounds in multiple ways. They rob calories from your energy stores and send them to fat cells. So not only are you adding pounds but your energy is being compromised, which drives up your cravings. Some people gain as many as 28 pounds on steroids. Ask about switching to prescription-strength NSAIDs (nonsteroidal anti-inflammatory drugs) such as ibuprofen.
If you gain weight due to medication, the key is patience. "When you go off the drug, you won't lose weight as fast as you gained it," says Aronne. "But by taking control of this aspect of your treatment, you'll start to see results."
Sara Reistad-Long is a writer living in New York. She has written for Esquire and Real Simple.