13 Drugs That Can Make You Gain Weight

About 70% of people in the United States are overweight and, in a cruel catch-22, many of the drugs used to treat obesity-linked conditions such as diabetes, high blood pressure, and depression can themselves cause weight gain.

"Patients and doctors need to be more aware of this—it's an under-recognized driver of our obesity problem," says Lawrence Cheskin, MD, director of the Johns Hopkins Weight Management Center, in Baltimore.

Here are 13 drugs that could cause you to gain weight. But don't stop taking your (possibly life-saving) medicine! There are strategies for keeping off the pounds.

Paxil (paroxetine)

Selective serotonin reuptake inhibitors (SSRIs) generally don't cause weight gain because the antidepressants boost serotonin, which helps you feel full. Paxil is an exception.

Paxil is one of the best anxiety treatments, but if you gain weight while using it you could talk with a doctor about switching to a more weight-neutral SSRI such as Zoloft, says Louis Aronne, MD, director of the Comprehensive Weight Control Program at New York-Presbyterian/Weill Cornell Medical Center, in New York City. (Dr. Aronne is a consultant for the maker of Paxil.)

Depakote (valproic acid)

Depakote is used to treat bipolar disorder and seizures, and prevent migraines.

A 2007 study of epilepsy patients found that 44% of women and 24% of men gained 11 pounds or more while taking Depakote for about a year. The drug affects proteins involved in appetite and metabolism, although it's not clear why it appears to affect women more than men.

Lithium, another mood stabilizer for treating bipolar disorder, is also associated with weight gain, albeit less than Depakote.

Prozac (fluoxetine)

Although Prozac, an SSRI, is generally associated with weight loss, it can have the opposite effect in the long term.

A 60-week study found that, although patients on Prozac shed more than the placebo group (up to 11 pounds in the first six months), they started to regain the weight about halfway through the study.

Dr. Cheskin says the reason could be that patients develop tolerance to the satiety effect. "If someone is trying to lose weight, and is on an SSRI for depression, I will often switch to Wellbutrin (bupropion)," which is associated with weight loss, he says.

Remeron (mirtazapine)

Remeron is an antidepressant that enhances serotonin and norepinephrine, which are linked to weight loss. Yet the antihistamine activity of this drug may tip the scales toward weight gain.

This side effect can be a good thing, however. Remeron is sometimes given to elderly people who are underweight to improve their appetite, Roerig says.

Zyprexa (olanzapine)

Atypical antipsychotics, such as Zyprexa and Clozaril (clozapine), can pile on the pounds. A 2005 study found that 30% of people on Zyprexa gained 7% or more of their body weight within 18 months.

These drugs, used for schizophrenia and bipolar disorder, have potent antihistamine activity and inhibit serotonin, which may trigger weight gain.

The diabetes drug metformin may help keep weight off, as can switching to weight-neutral antipsychotics like Geodon (ziprasidone) and Abilify (aripiprazole).

Deltasone (prednisone)

Oral corticosteroids, such as Deltasone, are more potent than inhaled forms and carry a higher risk of weight gain, particularly with long-term use.

A 2006 survey of long-term oral-corticosteroid users suggested 60% to 80% had gained weight. Higher doses, such as those for asthma and inflammatory bowel disease, are more likely to have this effect than lower doses, such as those for rheumatoid arthritis.

It can be hard to counteract the side effects of long-term steroid use, but exercise, diet, and, in some cases, taking metformin can help, Dr. Aronne says.

Thorazine (chlorpromazine)

When the first-generation antipsychotic Thorazine entered the market in 1954, it was clear that it could cause weight gain.

Thorazine, along with Mellaril (thioridazine), has antihistamine activity, which increases your appetite and is sedating, says James Roerig, associate professor of clinical neuroscience at the University of North Dakota School of Medicine and Health Sciences, in Fargo.

These drugs may cause less weight gain than Zyprexa and Clozaril. Still, they aren't as effective at treating schizophrenia, and may have other side effects, such as motor problems, says Roerig. (Roerig gets research funding from Eli Lilly, the maker of Zyprexa.)

Elavil, Endep, Vanatrip (amitriptyline)

Tricyclic antidepressants (TCAs), such as amitriptyline, are linked to weight gain more than other antidepressants or migraine meds.

TCAs affect neurotransmitters involved in energy and appetite, such as serotonin, dopamine, and acetylcholine, but their antihistamine activity is probably the reason for weight changes, Roerig says.

These drugs are given less often for depression because an overdose can damage the heart. Lower doses can prevent migraines, but a 2006 study found that 30% of people taking amitriptyline for migraines gained more than 5% of their body weight in 26 weeks.

Allegra (fexofenadine and pseudoephedrine)

The antihistamine activity in psychiatric drugs often is what causes weight gain, but it turns out that this property is not important for these drugs to be effective, Roerig says.

Antihistamine activity, however, is crucial for the effectiveness of allergy drugs. A 2010 study found that people taking prescription antihistamines such as Allegra and Zyrtec were 55% more likely to be overweight than those not taking the drugs.

Blocking histamine can disrupt an enzyme in the brain that helps regulate food consumption.

Diabinese, Insulase (chlorpropamide)

Some type 2 diabetes drugs cause weight loss. Others have the opposite effect.

Sulfonylurea drugs such as Diabinese and Insulase—and others such as Actos and Prandin—stimulate insulin production or activity, which lowers blood sugar and may increase appetite.

Metformin, Byetta, and Januvia are more likely to cause weight loss, Dr. Aronne says. (Dr. Aronne receives research support from and consults for the companies that make Byetta and other drugs.)

Insulin

Insulin tends to promote weight gain. But certain types, such as the long-acting insulin Levemir, have less extreme effects.

One study found that people gained nearly 11 pounds on average during their first three years taking insulin. About half of the weight gain is thought to occur in the first three months.

Dr. Cheskin calls the weight-promoting effect of insulin paradoxical. "The things that we use to treat some conditions that are the result of obesity, like [type 2] diabetes, are prone to make you more obese," he says.

Tenormin (atenolol)

Beta-blockers are among the go-to drugs for high blood pressure, but the older ones, such as Tenormin, Lopressor (metoprolol), and Inderal (propranolol), can expand the waistline.

One study found that people taking Tenormin gained about 5 more pounds than the placebo group, and research suggests that most of the weight is gained in the first few months. These drugs can slow calorie burning and cause fatigue.

Newer beta-blockers, calcium channel blockers, and ACE inhibitors are less likely to cause weight gain, Dr. Cheskin says.

Birth control

Birth-control pills are often blamed for weight gain, but it is largely unwarranted, Dr. Cheskin says.

In fact, only the long-acting, progestin-only injectable called depot medroxyprogesterone acetate (DMPA) has been consistently linked with weight gain.

Research suggests that young women who were obese to begin with are most prone to weight gain on DMPA.