Side Effects of Weight Loss Drugs

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Feb 3, 2025.

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

• GLP-1 receptor agonists such as liraglutide (Saxenda), semaglutide (Wegovy, Ozempic) and GIP and GLP-1 receptor agonists like tirzepatide (Zepbound, Mounjaro) can often cause stomach side effects like diarrhea, upset stomach (nausea or vomiting), heartburn, gas or constipation, among other side effects. 
• Stimulant-type drugs like phentermine (Adipex-P) can lead to insomnia, increased blood pressure, fast heart rate, restlessness, drug dependence, abuse, and withdrawal symptoms.
• Drugs that interfere with fat absorption, such as orlistat (Alli), can lead to oily spotting, gas, and soft stools.
• Diet pills that affect neurotransmitters in the brain, such as bupropion and naltrexone (Contrave) can be linked with headache, dry mouth, and dizziness, as well as stomach side effects like nausea, vomiting and constipation.

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 or more 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

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, or higher, has been seen with these newer treatments, but some people need to stop treatment because of stomach side effects (nausea, vomiting, diarrhea, constipation and stomach pain).
• Usually the stomach side effects, which are common, subside after the first few weeks and as you reach your final maintenance dose.

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. 

• Zepbound was first approved in November 2023 and is a GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptor agonist.
• It is used in addition to diet and exercise for long-term weight management in adults with obesity, or overweight with at least one weight-related medical problem (for example: hypertension, high cholesterol, type 2 diabetes, obstructive sleep apnea or heart disease).
• Zepbound was also approved for obstructive sleep apnea (OSA) in December 2024 for adults with moderate-to-severe OSA and obesity.
• The most common side effects (incidence ≥ 5%) include nausea, diarrhea, vomiting, constipation, stomach-area pain, heartburn, injection-site reactions, sleepiness, allergic reactions, burping, hair loss and acid reflux disease.
• Zepbound contains the same active ingredient as Mounjaro, approved to help control blood sugar in patients with type 2 diabetes.  

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

• Wegovy was first approved in June 2021 and is classified as a glucagon-like peptide-1 (GLP-1) receptor agonist.
• It is used in addition to diet and exercise for long-term weight management in:
  • adults with obesity
  • adults with overweight and with at least one other weight-related medical issue
  • children aged 12 years and older with obesity. 
• Wegovy is also approved to help protect the heart. It used to reduce the risk of heart attacks, strokes, and cardiovascular death in adults with obesity or overweight and cardiovascular (heart) disease.
• The most common side effects (incidence ≥ 5%) may include: nausea, diarrhea, vomiting, constipation, stomach-area pain, headache, tiredness (fatigue), heartburn, dizziness, feeling bloated, belching, low blood sugar in patients with type 2 diabetes, gas, stomach infection (gastroenteritis), acid reflux disease, and the common cold symptoms.
• It contains the same active ingredient as found in Ozempic, which is approved to help control blood sugar in patients with type 2 diabetes, for cardiovascular risk reduction in patients with heart disease, and to reduce the risk of serious kidney problems in adults with type 2 diabetes (T2D) and chronic kidney disease (CKD).

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. 

• Saxenda is a glucagon-like peptide-1 (GLP-1) receptor agonist first approved in Dec. 2014. It is used in addition to diet and exercise for long-term weight management in adults who are obese (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with other weight-related medical issues. It is also approved for use in children aged 12 years and older with body weight above 60 kg and an initial BMI corresponding to 30 kg/m² for adults (obese).
• Commonly reported side effects (incidence ≥ 5%) include: nausea, diarrhea, constipation, vomiting, injection site reactions, headache, low blood sugar, heartburn, fatigue (sleepiness), dizziness, stomach-area pain, increased lipase (a pancreatic enzyme), upper stomach-area pain, fever, and stomach infection (gastroenteritis).
• The Victoza brand of liraglutide contains the same active ingredient as Saxenda but is used to treat type 2 diabetes. It is not approved for weight loss.

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)

• Bupropion is an antidepressant medicine and naltrexone is usually given to block the effects of narcotics or alcohol in people with dependence. Both drugs may also curb hunger and food cravings.
• Contrave side effects can include constipation, dizziness, headache, trouble sleeping, and upset stomach. It is not classified as a controlled substance.

Qsymia (phentermine and topiramate ER)

• Phentermine suppresses the appetite similar to an amphetamine stimulant. Topiramate is used a medication to control seizures. These drugs, when used together with diet and exercise, can lead to weight loss, but the exact way this occurs is not fully known.
• Common side effects may include: paraesthesia (numbness or tingling sensation), dizziness, dysgeusia (abnormal taste), trouble sleeping, constipation, and dry mouth.
• Qsymia is classified as a CIV controlled substance due to the possibility of dependence (addiction) or abuse.

Stimulants

Side effects of stimulants may include:

• increased blood pressure and heart rate
• insomnia (trouble sleeping)
• nervousness
• blurred vision
• restlessness
• headache

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:

• diethylpropion
• benzphetamine
• methamphetamine (Desoxyn)
• phentermine (Adipex-P)
• phendimetrazine (Bontril PDM)
• phentermine and topiramate ER (Qsymia)

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.

• Xenical (orlistat): prescription only

• Alli (orlistat): over-the-counter (OTC), no prescription required. Alli is the over-the-counter version of Xenical, but it comes in a lower 60 milligram (mg) strength.

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:

GLP-1 agonist and GLP-1 / GIP agonist drugs like Saxenda (liraglutide), Wegovy (semaglutide) and Zepbound (tirzepatide) all contain a boxed warning for thyroid C-cell tumor cancer; however, studies have only identified this risk in animals.

• In studies with rats and mice, these medicines caused thyroid tumors, including thyroid cancer. It is not known if these medicines will cause thyroid tumors or a type of thyroid cancer called medullary thyroid carcinoma (MTC) in people.
• If you have a personal or family history of medullary thyroid carcinoma (MTC) or an endocrine system condition called Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), you should NOT use these medicines due to the possible risk of thyroid tumors.
• Tell your healthcare provider if you get a lump or swelling in your neck, hoarseness, trouble swallowing, or shortness of breath, as this may be a symptom of thyroid cancer.

These products may also lead to inflammation (swelling) of your pancreas (pancreatitis), gallbladder problems, kidney problems, vision problems (diabetic retinopathy), an increased risk of low blood sugar, or suicidal thoughts or behavior.

In November 2024, the FDA updated the warnings for all GLP-1 agonist and GLP-1 / GIP agonist medicines to note that these agents can increase the risk of food or liquid getting into your lungs (called pulmonary aspiration) during procedures or surgery while you are under deep sedation or general anesthesia. This can occur because these medicines can slow down gastric emptying. Tell all your healthcare providers that you are taking a GLP-1 and GLP-1 / GIP weight loss drug before you are scheduled to have surgery or any other procedure.

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.

• Qsymia has restricted access because one of its active ingredients known as topiramate may cause severe birth defects in pregnant women. This means you may only be able to get Qsymia from certified pharmacies or through the mail by the Qsymia Home Delivery network.
• Do not use Qsymia if you are pregnant or planning a pregnancy. Women must have a negative pregnancy test before starting Qsymia and each month while taking the medication.

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

• Contrave’s label contains a boxed warning found on all antidepressants: a risk of suicidal thoughts and actions in children, adolescents, and young adults when used for depression.
• Do not use Contrave with other bupropion-containing products (such as Wellbutrin SR, Wellbutrin XL, Aplenzin) as it may increase the risk of severe side effects. Do not use if you have a risk for seizures.
• Serious psychiatric side effects, like mood changes (depression, mania), psychosis, hallucinations, paranoia, delusions, thoughts of suicide, and other serious events have been reported in patients using bupropion for smoking cessation.
• May cause liver damage; avoid alcohol use with Contrave.

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 your weight loss drug read the product information and discuss this information with your doctor or other healthcare provider.

Safety issues with discontinued weight loss drugs 

• Fenfluramine (Pondimin), used in the combination drug fen-phen (fenfluramine and phentermine) was associated with dangerous heart valve side effects and is no longer available on the U.S. market due to this side effect. Fenfluramine inhibits serotonin reuptake to reduce appetite. In 1997, the FDA withdrew fenfluramine and dexfenfluramine (Redux) from the market, as well.
• Sibutramine (Meridia) was removed from the U.S. market in 2010, also due to an increased risk of serious heart side effects, including heart attack and stroke. Sibutramine blocks norepinephrine and serotonin reuptake to lead to the appetite suppressant effect.
• In 2020, the FDA requested that the manufacturer withdraw lorcaserin (Belviq, Belviq XR) from the U.S. market as the risk of cancer (including pancreatic, lung and colorectal) outweighed any benefit from the drug.

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 the stimulant ephedra completely from the market 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.

• Check with your healthcare provider first if you want to use a dietary supplement for weight loss.
• Review FDA recalls and Medwatch alerts to determine if there are safety issues if you choose to use a dietary supplement for weight loss.
• Always avoid buying questionable products from the Internet that may contain dangerous, unapproved, or counterfeit ingredients.

Bottom Line

• Newer prescription weight loss drugs like Wegovy or Zepbound can produce dramatic weight loss results of at least 20%, but come with a initial risk of stomach-related side effects, like nausea, vomiting, diarrhea or stomach-area pain. These side effects typically subside in a few weeks and once you reach your final maintenance dose.
• Drugs that are considered appetite suppressants and that contain stimulants may common cause insomnia (trouble sleeping), constipation, headache or dry mouth. Some of these drugs are classified as controlled substances due to a risk of dependence and abuse.
• Lipase inhibitors such as alli or Xenical (orlistat) can cause oily spotting, gas, stomach pain, fecal urgency or incontinence, soft stools, and the possibility of serious liver injury. alli is available over-the-counter (OTC) and does not require a prescription. 

This is not all the information you need to know about weight loss drugs or their side effects for safe and effective use and does not take the place of your doctor’s directions. Review the full product information and discuss this information and any questions you have with your doctor or other health care provider.

Further information

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

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