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 percent of people taking amitriptyline for migraines gained more than 5 percent 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 percent 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.

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