Diet Pills: Do They Really Work?

woman standing on a scale


Some do, some don't—and it's important to understand the difference. Currently, the U.S. Food and Drug Administration (FDA) has approved three drugs to aid long-term weight loss. They are: orlistat (Xenical and Alli), lorcaserin (Belviq) and phentermine-topiramate (Qsymia).

There also are many over-the-counter appetite suppressants and other drugs and supplements that promise rapid weight loss, but they have not proven effective for lasting weight loss, and some can cause potentially dangerous side effects.

The cornerstone of successful weight loss involves lifestyle changes, such as eating a healthful, high-fiber, lower-calorie diet, being physically active and modifying negative behaviors. Numerous studies prove the health dangers of excess pounds, so it's important to try to achieve and maintain a healthy weight.

If you are among the nearly 70 percent of U.S. adults age 20 and older who are overweight or obese, the prescription weight-loss medications may help you get to a healthier weight. However, they won't help everyone. Weight loss is a serious, long-term endeavor. Always discuss  your options with your health care provider or consult a registered dietitian.

No weight-loss medications should be used if you're pregnant or trying to become pregnant. Some medications may be restricted if you are less than 18 years old or have certain medical conditions. Like most medications, weight-loss medications have potential side effects. None of the long-term weight-loss drugs is considered high risk for developing a drug dependence, but you should take care if you have a history of drug or alcohol addiction.

Here's a primer on weight-loss medications:

  • Orlistat works by preventing your body from absorbing about one-third of the fat you eat. It can interfere with the absorption of fat-soluble vitamins, so you should take a multivitamin while taking orlistat. It can cause diarrhea, stomach  pain, gas and other gastrointestinal symptoms. These symptoms are usually mild and temporary but may be worse if you eat high-fat foods. In rare cases, orlistat may cause liver damage, so stop taking the drug and contact your health care professional if you notice any symptoms of liver problems, such as dark urine or yellowish eyes or skin. Xenical is the prescription version of orlistat, approved for adults and children 12 and older. Alli is the lower-dose over-the-counter orlistat. It is not approved for children. Do not take orlistat while taking cyclosporine.
  • Lorcaserin affects the chemicals in your brain that help decrease your appetite and make you feel full, so you eat less. It does not work for all people, so if you do not lose 5 percent of your weight within 12 weeks of starting the drug, the medication may not work for you, and you should talk with your health care professional about other options. Side effects of lorcaserin may include headaches, dizziness, fatigue, nausea, dry mouth, cough and constipation. If you are also taking an SSRI antidepressant or MAOI, ask your health care professional about the risk of serotonin syndrome.
  • Phentermine-topiramate combines two FDA-approved drugs. The phentermine suppresses appetite, but used alone, it is only approved for two weeks. By combining a lower dosage of phentermine with extended-release topiramate, the drug has been shown to be safe for up to two years. The medication comes in three doses: starting dose, recommended dose and higher dose. Similar to lorcaserin, it is not effective for everyone, so if you have not lost at least 5 percent of your body weight after 12 weeks on the higher dose of phentermine-topiramate, talk to your health care professional about other options. Common side effects may include tingling hands and feet, dizziness, changes in taste, trouble sleeping, constipation and dry mouth. Rare side effects may include allergic reactions, memory or mood problems, suicidal thoughts, vision problems and kidney stones. It is not recommended if you have an overactive thyroid gland, glaucoma or have recently taken an MAOI antidepressant (though it was safely tested with other antidepressants).

There also are several over-the-counter FDA-approved appetite suppressants that affect the brain chemicals that regulate your appetite. They include:

  • phentermine (Adipex-P, Oby-Cap, Suprenza, T-Diet, Zantryl)
  • benzphetamine (Didrex)
  • diethylpropion (Tenuate, Tenuate Dospan)
  • phendimetrazine (Adipost, Bontril PDM, Bontril Slow Release, Melfiat)

These medications have the potential for abuse and are only FDA-approved for short-term use of up to 12 weeks, though some health care professionals may prescribe them for longer "off-label" use. Side effects can include dry mouth, difficulty sleeping, dizziness, headache, nervousness, restlessness, upset stomach and diarrhea or constipation. Severe side effects may include chest pain, fainting, rapid heartbeat, shortness of breath, confusion and swelling in your ankles or feet. You should not take appetite suppressants if you have heart disease, high blood pressure, overactive thyroid gland or glaucoma.

These appetite suppressants, along with other over-the-counter remedies like diuretics and supplements, have not been found to be effective long term and some have been linked to serious side effects.

Some issues to think about before considering a weight loss medication:

  1. Your degree of overweight: weight loss medications are usually prescribed for people with a body mass index (BMI) over 27 who have additional complications, such as diabetes; or for those with BMIs over 30 without complications. BMI is a measure of weight status: people with BMIs from 25 to 29.9 are considered overweight; those with BMIs of 30 and above are considered obese.
  2. Your degree of success with other weight loss programs or methods.
  3. Your ability to comply with taking the medications and making long-term lifestyle changes.
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