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Side Effects of Weight Loss Drugs (Diet Pills)

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on June 22, 2021.

Side effects with weight loss drugs can vary depending upon the type of drug you take and how the drug works. See Table 1.

  • 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.
  • Glucagon-like peptide-1 (GLP-1) receptor agonists such as liraglutide (Saxenda) or semaglutide (Wegovy) can often cause stomach side effects like diarrhea, upset stomach (nausea or vomiting), heartburn, gas or constipation, among other side effects.
  • 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.

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. However, 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.

Prescription weight loss drugs such as phentermine and topiramate (Qsymia), bupropion and naltrexone (Contrave), and liraglutide (Saxenda) 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.

Weight loss medications are used alongside a low calorie diet and a doctor-approved exercise plan for best results. You may see a 3% to 9% weight loss with the use of some diet drugs. However, it’s important to know that most 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?

FDA Approved Weight Loss Drugs

Anorexiants (Appetite Suppressants)

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.


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.

These drugs are also classified as controlled substances. As these drugs are amphetamine-related, 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:

Newer Weight Loss Medicines

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, difficulty sleeping, and upset stomach. Contrave is not classified as a controlled substance.

Saxenda (liraglutide)

  • Saxenda is a hormone injection (a glucagon-like peptide-1 (GLP-1) receptor agonist) that works in the brain to help control blood sugar, insulin levels, and digestion, but it also may also help with weight loss.
  • Commonly reported side effects include: upset stomach, low blood sugar, diarrhea, constipation, headache, fatigue, dizziness, and increased lipase.
  • The Victoza brand of liraglutide is used together with diet and exercise to treat type 2 diabetes and is NOT used for weight loss. Saxenda and Victoza should never be used together.
  • Saxenda 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), difficulty sleeping, constipation, and dry mouth.
  • Qsymia is classified as a CIV controlled substance due to the possibility of dependence (addiction).

Wegovy (semaglutide) is a once-weekly subcutaneous injection. It is not a controlled substance.

  • 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 who are overweight (BMI ≥27 kg/m2) or obese (BMI ≥30 kg/m2).
  • The most comm side effects are may include: nausea, vomiting, stomach (abdomen) pain, dizziness, stomach flu, diarrhea, headache, feeling bloated, heartburn, tiredness (fatigue), belching, constipation, upset stomach, and gas.

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. Alli 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.

Table 1. Common Side Effects of 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, decreased appetite, dizziness, headache, heartburn, fatigue, dizziness, stomach pain, gas, dry mouth, low blood sugar in type 2 diabetes, elevated heart rate, increased lipase.
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.

What are the serious side effects with weight loss pills (diet pills)?

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

  • 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).
  • 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.
  • 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).
  • Qsymia (phentermine and topiramate) is an FDA-approved extended-release weight loss medication.
    • Qsymia has restricted access because topiramate, one of the drugs contained in Qsymia, can lead to 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.
  • Saxenda (liraglutide) and Wegovy (semaglutide) both 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.
  • Saxenda or Wegovy may cause inflammation (swelling) of your pancreas (pancreatitis), gallbladder problems, kidney problems and an increased risk of low blood sugar.

This is not a complete list of all common or serious side effects. There are many 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.

Discontinued Weight Loss Drugs in the U.S.

  • 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?

Some, but not all, insurance plans cover medications that 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.

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 not true.

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.

  • 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.

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Further information

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