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Side Effects of Weight Loss Drugs

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Nov 14, 2023.

Side effects of drugs used for weight loss may vary depending upon the type of drug you take and how the drug works. See the table of side effects below.

Overview of side effects

The latest prescription drugs, like Zepbound, Wegovy and Saxenda are approved for chronic (long-term) weight loss and come as subcutaneous (under the skin) injections. Up to a 20% weight loss has been seen with these newer treatments. But digestive tract symptoms commonly occur and may lead some people to stop treatment. 

Prescription weight loss drugs such as phentermine and topiramate (Qsymia), bupropion and naltrexone (Contrave) are used for chronic, long-term weight loss, but only if adequate results occur. In general, if a 3% to 4% weight loss has not been achieved after 12 to 16 weeks, long-term treatment is typically stopped.

Many stimulant-type weight loss medications like phentermine or diethylpropion are only recommended for short-term use (up to 12 weeks) due to risk of dependence and other side effects.

Orlistat (alli, Xenical) can be used for longer-term weight loss, including the maintenance of previously lost weight. You can also get alli at the drugstore without a prescription.

Weight loss medications should be used alongside a low calorie diet and a doctor-approved exercise plan for best results. In most cases, people will regain some or all of the weight they lost when they stop using weight loss drugs unless diet and exercise are continued.

Be sure to talk to your doctor for sound advice before starting any weight loss program and understand it will take time and discipline for permanent results. If you stop taking your weight loss medication, continue with your diet and exercise program, as directed by your physician.

Table of Common Side Effects with Weight Loss Drugs

Brand Drug Name Drug Category Common Side Effects
Adipex-P (phentermine) Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use. When given as a single agent, phentermine is not implicated in valvular heart disease; avoid use at bedtime.
alli (orlistat): over-the-counter (OTC) Lipase inhibitor Oily spotting, gas (flatulence), fecal urgency, soft stools, fecal incontinence; take Alli with a daily multivitamin once-a-day at bedtime.
Bontril PDM (phendimetrazine) Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use; decreased appetite, avoid use at bedtime.
Contrave (bupropion and naltrexone) Antidepressant (weak inhibitor of norepinephrine and dopamine) and an opioid antagonist Nausea, vomiting, headache, fatigue, constipation, dizziness, difficulty sleeping, dry mouth, diarrhea, increased blood pressure, fast heart rate anxiety, tremor, hot flush, unusual taste.
Desoxyn (methamphetamine) Appetite suppressant; sympathomimetic amine High abuse potential and not frequently prescribed; use only if alternative treatments are ineffective; increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use; avoid use at bedtime.
benzphetamine Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use; decreased appetite, avoid use at bedtime.
diethylpropion (generic only) Appetite suppressant; sympathomimetic amine Constipation, restlessness, dry mouth, increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use; avoid use at bedtime.
Qsymia (phentermine and topiramate extended-release capsules) Combination appetite suppressant-seizure drug; exact action of topiramate on weight loss is not known Tingling or numbing sensation (paresthesias), dizziness, altered taste, insomnia (difficulty sleeping), constipation, dry mouth. The dose is taken once daily in the morning; avoid an evening dose as it may keep you awake. Do not use if pregnant or planning a pregnancy due to possible birth defects; avoid use at bedtime.
Saxenda (liraglutide) Glucagon-like peptide-1 (GLP-1) receptor agonist; regulates appetite and food intake; once-daily subcutaneous injection.

Nausea and vomiting, diarrhea, constipation, headache, dizziness, injection site reactions, heartburn, fatigue, dizziness, stomach-area pain, fever, and stomach infection (gastroenteritis).

Xenical (orlistat) Lipase inhibitor Oily spotting, gas (flatulence), fecal urgency, soft stools, fecal incontinence; take Xenical with a daily multivitamin containing fat-soluble vitamins.
Wegovy (semaglutide) Glucagon-like peptide-1 (GLP-1) receptor agonist; regulates appetite and food intake; once-weekly subcutaneous injection. Nausea, vomiting, stomach pain, dizziness, stomach flu, diarrhea, headache, feeling bloated, heartburn, tiredness (fatigue), belching, constipation, upset stomach, and gas.
Zepbound (tirzepatide) GIP and GLP-1 (glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1) receptor agonist; once weekly subcutaneous injection Nausea and vomiting, diarrhea, constipation, stomach-area pain, heartburn, injection-site reactions, sleepiness (fatigue), allergic reactions, burping / belching, hair loss and acid reflux disease.

The latest prescription drugs, like Zepbound, Wegovy and Saxenda are approved for chronic (long-term) weight loss and come as subcutaneous (under the skin) injections. Up to a 20% weight loss has been seen with these newer treatments. But digestive tract symptoms commonly occur and may lead some people to stop treatment. 

Prescription weight loss drugs such as phentermine and topiramate (Qsymia), bupropion and naltrexone (Contrave) are used for chronic, long-term weight loss, but only if adequate results occur. In general, if a 3% to 4% weight loss has not been achieved after 12 to 16 weeks, long-term treatment is typically stopped.

Many stimulant-type weight loss medications like phentermine or diethylpropion are only recommended for short-term use (up to 12 weeks) due to risk of dependence and other side effects.

Orlistat (alli, Xenical) can be used for longer-term weight loss, including the maintenance of previously lost weight. You can also get alli at the drugstore without a prescription.

Weight loss medications should be used alongside a low calorie diet and a doctor-approved exercise plan for best results. In most cases, people will regain some or all of the weight they lost when they stop using weight loss drugs unless diet and exercise are continued.

Be sure to talk to your doctor for sound advice before starting any weight loss program and understand it will take time and discipline for permanent results. If you stop taking your weight loss medication, continue with your diet and exercise program, as directed by your physician.

Learn More: FDA-Approved Weight Loss Drugs: Can They Help You?

Weight Loss Drugs Approved by the FDA

Incretin Mimetics

Zepbound (tirzepatide) is a once weekly subcutaneous (under the skin) injection. 

Wegovy (semaglutide) is a once-weekly subcutaneous (under the skin) injection. 

Other incretin mimetics, like Ozempic (semaglutide) and Mounjaro (tirzepatide), approved for blood sugar control in type 2 diabetes, are often used "off-label" for weight loss, meaning they may be prescribed by your doctor for a generally accepted use not specifically approved by the FDA or listed in package labeling.

Saxenda (liraglutide) is a subcutaneous (under the skin) injection given once a day. 

These weight loss agents should not be used in combination or with each other or any other GLP-1 or GIP / GLP-1 receptor agonists.

These medicines are not classified as controlled substances.

Anorexiants (Appetite Suppressants / Stimulants)

Anorexiants are drugs that act in the brain to help lower a person’s appetite. They have an effect on the parts of the brain which help control how full you feel. Anorexiants are used as a treatment for weight loss, along with a regular plan of diet and exercise. There are different types of appetite suppressants, including amphetamine-like stimulants and new weight loss drugs that act on neurotransmitters in the brain.

Contrave (bupropion and naltrexone)

Qsymia (phentermine and topiramate ER)

Stimulants

Side effects of stimulants may include:

Some forms may cause stomach side effects like constipation, dry mouth, nausea or vomiting. A healthcare provider should be contacted if side effects like chest pain, pounding or rapid heartbeat, difficulty urinating, or shortness of breath occur.

Stimulants are classified as controlled substances. As these medicines are amphetamine-related, drug dependence, abuse or withdrawal may occur with long-term use. Keep them in a secure place away from children and pets.

Examples of stimulant weight-loss drugs include:

Lipase Inhibitors

Xenical, the prescription form of orlistat, was first FDA-approved in 1999. Xenical was followed by the lower dose, over-the-counter (OTC) orlistat (alli) in 2007. Orlistat, a lipase inhibitor, is unique in that it does not affect the central nervous system to induce weight loss, but instead acts peripherally to help prevent absorption of fat from the food that you eat.

Side effects of orlistat can be unpleasant: oily spotting, gas, stomach pain, fecal urgency or incontinence, soft stools, and the possibility of serious liver injury can occur. Take alli or Xenical with a daily multivitamin that contains fat-soluble vitamins such A, D, E, K and beta carotene, as they may reduce the absorption of fat soluble vitamins.

What are the serious side effects with weight loss drugs?

Weight loss drugs can be associated with important or serious side effects, and some may have the potential for dependence and abuse. Important warnings and side effects include:

Amphetamine-derivative, stimulant type weight loss drugs may be associated with constipation, dry mouth, restlessness, withdrawal effects, or insomnia (difficulty falling asleep), drug abuse and addiction. Many of the weight loss drugs known as sympathomimetic amines can stimulate the heart and lead to high blood pressure and a fast heart rate (tachycardia).

Qsymia (phentermine and topiramate) is an FDA-approved extended-release weight loss medication.

Contrave (bupropion and naltrexone) is not used for depression but contains bupropion from the antidepressant class of drugs.

Lipase inhibitors such as alli or Xenical (orlistat) have been associated with rare liver injury and patients should be alert for signs of liver disease such as itching, yellow skin or eyes (jaundice), stomach pain, loss of appetite, pale or tar-colored stools, brown-colored urine (due to excess bilirubin in the urine).

This is not a complete list of all common or serious side effects. There are other potentially serious side effects. For a full listing of all side effects linked with weight loss drugs, read each monograph individually and discuss with your doctor.

Safety Issues with Discontinued Weight Loss Drugs 

Will Insurance Pay For Weight Loss Medication?

At this time, most insurance plans in the US do not cover medications to treat overweight and obesity. Contact your insurance provider to find out if your plan covers these medications.

Be aware that the over-the-counter medication alli will not be covered by your insurance, but Xenical may be, so be sure to check and discuss this option with your doctor.

Manufacturers may offer copay card assistance, especially with the newer agents like Zepbound or Wegovy, which may help to save you money if you have commercial insurance. Contact the manufacturer for further information.

Are Herbal Weight Loss Drugs Safe?

If you are interested in losing weight, you may have considered buying an”all-natural” or “herbal” diet weight loss product, also called a weight loss dietary supplement, at the store or from the Internet. Chromium, Green Tea extract, Hoodia, and Guar Gum are some of the herbal dietary supplements available on the market today that claim to help with weight loss. These products may claim to "magically melt" fat away and seem like an easy fix to shed a few pounds. They might claim you will “lose 10 pounds in one week”, but these claims are false.

Federal regulators have warned that many of these marketed dietary supplements aren’t effective and may even cause serious side effects. Authorities have found products that contain prescription drugs like sibutramine, withdrawn from the U.S. market in October 2010 due to safety concerns. In 2004 the FDA removed dietary products with ephedra, a stimulant, from the market completely due to severe side effects such as heart attack, stroke and seizures. Other ingredients discovered by the FDA include phenolphthalein, blood pressure treatments, seizure medications, and other mislabeled products.

It’s important to know that over-the-counter (OTC) dietary supplements are not reviewed or approved by the FDA. The FDA does investigate herbal supplements if they appear to be causing harm. Almost every week, the FDA removes tainted dietary supplements from the market due to health concerns.

Many examples exist of recalled dietary supplements used for weight loss and other conditions. Be on the safe side: if it sounds too good to be true -- it probably is.

Sources

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.