What drugs are approved in the U.S. for weight loss?

Four prescription weight-loss drugs are approved by the U.S. Food and Drug Administration (FDA) for long-term use: bupropion-naltrexone (Contrave), liraglutide (Saxenda), orlistat (Xenical) and phentermine-topiramate (Qsymia).

Most prescription weight-loss drugs work by decreasing appetite or increasing feelings of fullness, and some do both. The exception is orlistat, which works by interfering with absorption of fat.

Bupropion-naltrexone is a combination drug. Naltrexone is used to treat alcohol and opioid dependence. Bupropion is an antidepressant and quit-smoking aid. Like all antidepressants, bupropion carries a warning about suicide risk. Bupropion-naltrexone can raise blood pressure, and monitoring is necessary at the start of treatment. Common side effects include nausea, headache and constipation.

Liraglutide is also used to manage diabetes. Unlike other weight-loss drugs, liraglutide is administered by injection. Nausea is a common complaint, and vomiting may limit its use.

Orlistat can cause bothersome gastrointestinal side effects, such as flatulence and loose stools. It's necessary to follow a low-fat diet when taking this medication. Orlistat is also available in a reduced-strength form without a prescription (Alli). Rare cases of serious liver injury have been reported. However, no cause-and-effect relationship has been established.

Phentermine-topiramate is a combination of an anticonvulsant (topiramate) and a weight-loss drug (phentermine). Phentermine has the potential to be abused because of its amphetamine-like effects. Other possible side effects include an increase in heart rate and blood pressure, insomnia, constipation and nervousness. Topiramate increases the risk of birth defects.

Phentermine as a single agent (Adipex-P) is also used for weight loss. It's one of four weight-loss drugs approved for short-term use (less than 12 weeks). This group of drugs isn't widely prescribed because of the limited duration of use, side effects and potential for abuse. The exception is phentermine. It's commonly prescribed and the actual rate of abuse appears to be low.