Common weight-loss drugs

Diethylpropion (Tenuate) and phentermine (Adipex) are approved by the Food and Drug Administration (FDA) for only short-term use — generally less than 12 weeks. Orlistat (Xenical), lorcaserin (Belviq) and a combination drug (Qsymia) are approved by the FDA for long-term use. Qsymia contains phentermine and an extended-release (ER) form of topiramate. Orlistat is also available in a reduced-strength form without a prescription (Alli).

The chart shows the commonly used prescription weight-loss drugs, how they work and their side effects.

Drug Mechanism of action Possible side effects
Benzphetamine (Didrex) Decreases appetite, increases feeling of fullness Increased blood pressure and heart rate, nervousness, insomnia, dry mouth, constipation
Diethylpropion (Tenuate) Decreases appetite, increases feeling of fullness Headache, increased blood pressure and heart rate, nervousness, insomnia, dry mouth, constipation
Lorcaserin (Belviq) Decreases appetite, increases feeling of fullness Headache, nausea, dry mouth, dizziness, fatigue, constipation
Naltrexone and bupropion extended-release (Contrave) Decreases appetite, increases feeling of fullness Nausea, constipation, headache, vomiting, dizziness
Phendimetrazine (Bontril) Decreases appetite, increases feeling of fullness Increased blood pressure and heart rate, nervousness, insomnia, dry mouth, constipation
Phentermine (Adipex, Suprenza) Decreases appetite, increases feeling of fullness Headache, increased blood pressure and heart rate, nervousness, insomnia, dry mouth, constipation
Orlistat (Xenical) Blocks absorption of fat Decreased absorption of fat-soluble vitamins, oily spotting, intestinal cramps, gas with discharge, diarrhea, fecal urgency and incontinence
Phentermine and extended-release topiramate (Qsymia) Decreases appetite, increases feeling of fullness Insomnia, dry mouth, dizziness, constipation, pins and needles feeling, changes in sense of taste or smell
Liraglutide (Saxenda) Slows gastric emptying, increases feeling of fullness Nausea, vomiting, pancreatitis

Safety of weight-loss drugs

Diethylpropion (Tenuate), phentermine (Adipex-P), benzphetamine (Didrex) and phendimetrazine (Bontril) are approved for only short-term use — generally less than 12 weeks. These drugs are classified as controlled substances because they have the potential to be abused. Because of possible side effects, these drugs aren't recommended if you have heart disease, high blood pressure or hyperthyroidism.

Orlistat (Xenical), lorcaserin (Belviq), phentermine-topiramate (Qsymia), naltrexone-bupropion (Contrave) and liraglutide (Saxenda) are approved for long-term use. Orlistat is also available in a reduced-strength form without a prescription (Alli). Limiting your intake of dietary fat is critical when taking orlistat.

After orlistat was approved, rare cases of serious liver injury were reported in some people taking it. No cause-and-effect relationship was established between orlistat and liver injury. However, Xenical and Alli labels now advise people taking orlistat to be alert to signs and symptoms that could indicate liver injury, such as itching, loss of appetite, yellow eyes or skin, light-colored stool, or brown urine.

Lorcaserin (Belviq) initially raised concerns because it works somewhat like fenfluramine — which was withdrawn from the market because it damaged heart valves. However, there is no evidence that Belviq damages heart valves. Belviq may increase heart rate, so people taking it may need to have their heart rate checked.

The combination drug Qsymia (phentermine and topiramate) increases the risk of birth defects. Thus, the Food and Drug Administration (FDA) required the manufacturer to have a risk evaluation and mitigation strategy (REMS). The REMS is intended to alert women to the risks and explain the need to avoid becoming pregnant while taking Qsymia. In addition, Qsymia is considered a controlled substance because one of its ingredients — phentermine — has the potential for abuse.

The combination drug Contrave contains naltrexone and bupropion. Naltrexone is used to treat alcohol and opioid dependence, while bupropion is an antidepressant that often reduces the risk of weight gain in people who are trying to quit smoking. Contrave also can raise heart rate and blood pressure and may increase the risk of seizures. Bupropion can increase the risk of suicidal thoughts and behaviors.

Liraglutide (Saxenda) is the newest drug to be approved for weight loss. The FDA approved it with the requirement that the manufacturer have a REMS to inform doctors about the serious risks associated with Saxenda. Saxenda has a boxed warning stating that tumors of the thyroid gland have been observed in animal studies but that it is unknown whether Saxenda causes these tumors in humans. Unlike the other weight-loss drugs, Saxenda is administered by injection once daily.

Factors to consider

If you meet the criteria for prescription weight-loss drugs, you and your doctor will need to evaluate the potential benefits against the possible risks of taking the medicine. Cost also is a consideration. Not all health insurance plans cover prescription weight-loss drugs.

As you consider weight-loss drugs, make sure that you make every effort to exercise, change your eating habits and adjust any other lifestyle factors that have contributed to your excess weight.

Weight-loss drugs aren't the easy answer to weight loss, but they can be a useful tool to help you make the necessary diet and lifestyle changes. Keeping off the pounds you've lost, however, is an ongoing concern. And many people, despite their efforts, still regain the weight.

Feb. 18, 2015 See more In-depth