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Side Effects of Weight Loss Drugs (Diet Pills)

Medically reviewed on Oct 21, 2016 by L. Anderson, PharmD


Side effects with weight loss drugs can vary depending upon the type of drug you take and how the drug works. Stimulants-type drugs like phentermine can lead to insomnia, increased blood pressure, fast heart rate, restlessness, drug dependence and 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 Belviq (lorcaserin) or Contrave (bupropion and naltrexone) can be linked with headache, nausea and vomiting, constipation, dry mouth, and dizziness.

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 get Alli at the drugstore without a prescription.

Newer prescription weight loss drugs such as Qsymia (phentermine and topiramate), Belviq (lorcaserin), Contrave (bupropion and naltrexone), and Saxenda (liraglutide) are indicated for chronic, long-term use, but only if adequate weight loss (as outlined in the package labeling) occurs. If a 3 to 4 percent 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. Plus, 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 efforts are made to maintain their weight loss with diet and exercise.

FDA Approved Weight Loss Drugs

Appetite Suppressants (anorexiants)

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 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, nervousness, blurred vision, restlessness, or headache. As these drugs are amphetamine-related, dependence, abuse or withdrawal may occur with long-term use. 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.

Examples of stimulants include:

Latest FDA Approved Weight Loss Drugs

  • Belviq (lorcaserin) - It is thought that Belviq helps to decrease appetite and increase the feeling of fullness by working at a certain serotonin receptor in the brain. Headache, nausea, dizziness, fatigue, dry mouth, constipation are common side effects. Low blood sugar can occur in patients with diabetes. Belviq is classified as a CIV controlled substance.
  • 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, but both drugs may also curb hunger and food cravings. Contrave is not classified as a controlled substance.
  • Saxenda (liraglutide) -  Saxenda is a hormone injection tht works in the brain to helps control blood sugar, insulin levels, and digestion, but it also may also help with weight loss. The Victoza brand of liraglutide is used together with diet and exercise to treat type 2 diabetes. Saxenda and Victoza should never be used together. Saxenda is not a controlled substance.
  • Qsymia (phentermine/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. Qsymia is classified as a CIV controlled substance.

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 is unique in that does not affect the central nervous system to induce weight loss, but instead acts peripherally to prevent absorption of fat. Side effects can be unpleasant: oily spotting, gas, fecal urgency, soft stools, fecal incontinence, 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.

Common Side Effects Associated 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.
Alli (orlistat) - over-the-counter (OTC) Lipase inhibitor Oily spotting, gas (flatulence), fecal urgency, soft stools, fecal incontinence; take Alli with a daily multlivitamin.
Belviq (lorcaserin) Selective serotonin 2C receptor agonist (suppresses appetite) Headache, nausea, dizziness, fatigue, dry mouth, constipation; euphoria/dissociation may occur with higher doses; do not exceed 10 mg two times a day.
Bontril PDM, Bontril SR (phendimetrazine) Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use.
Contrave (bupropion and naltrexone) Antidepressant (weak inhibitor of norepinephrine and dopamine) and an opioid antagonist Nausea, vomiting, headache, fatigue, constipation, dizziness, insomnia, dry mouth, diarrhea, increased blood pressure, anxiety, tremor, hot flush, unusual taste.
Desoxyn (methamphetamine) Appetite suppressant; sympathomimetic amine High abuse potential; 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.
Didrex (benzphetamine) Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use.
Diethylpropion 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.
Qsymia (phentermine and topiramate extended-release capsules) Combination appetite suppressant-anorectic; exact action of topiramate on weight loss is not known Paresthesias, dizziness, altered taste, insomnia, constipation, dry mouth.
Saxenda (liraglutide) Glucagon-like peptide-1 (GLP-1) receptor agonist; regulates appetite and food intake. Nausea/vomiting, diarrhea, constipation, headache, heartburn, fatigue, dizziness, stomach pain, gas, dry mouth, low blood sugar in type 2 diabetes, increased lipase.
Suprenza (phentermine) Appetite suppressant; sympathomimetic amine Increased blood pressure and heart rate, insomnia, nervousness, restlessness, dependence, abuse or withdrawal may occur with long-term use.
Xenical (orlistat) Lipase inhibitor Oily spotting, gas (flatulence), fecal urgency, soft stools, fecal incontinence; take Xenical with a daily multivitamin.

Serious side effects occur 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) was FDA-approved as a new extended-release weight loss medication in 2012. 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.
  • Contrave (bupropion and naltrexone), approved in 2014 for weight loss, 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. In addition, Contrave is also not approved for smoking cessation, but contains the same ingredient that is in Zyban, a medication used to help people stop smoking. 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.
  • Saxenda (liraglutide) contains a boxed warning for thyroid C-cell tumor cancer; however, studies have only identified this risk in animals at this point. However, if you have a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2, you should not use Saxenda due to the possible risk of thyroid tumors.

Banned Weight Loss Drugs in the U.S.

  • Fenfluramine (Pondimin), used in the combination drug fen-phen (fenfluramine/phentermine) was associated with dangerous heart valve side effects and is no longer available due to this side effect. Fenfluramine inhibits serotonin reuptake to reduce appetite. In 1997, the FDA withdrew fenfluramine and dexfenfluramine (Redux) from the U.S. market.
  • 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.

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.

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. These products may claim to "magically melt" fat away and seem like an easy fix to shed a few pounds. They claim you will “lose 10 pounds in one week”, but these products are fraudulent. It’s important to know that over-the-counter (OTC) dietary supplements are not FDA approved, either.

Federal regulators have warned that many of these marketed dietary supplements aren’t effective and may even cause serious side effects. Authorities have even found products that contain prescription drugs like sibutramine, withdrawn from the U.S. market in October 2010. The FDA notes that deaths have occurred with use of these dangerous products. Other ingredients discovered by the FDA include seizure medications, blood pressure treatments, and other drugs not FDA-approved.

Almost every week, the FDA removes tainted dietary supplements from the market due to health concerns. For example, in January 2016, the FDA issued an alert about “Pink Bikini and Shorts on the Beach Capsules” from Lucy's Weight Loss System of Arlington, TX. It was found that these weight loss aids contained sibutramine, phenolphthalein, and/or diclofenac, but these ingredients were not declared on the label. Sibutramine can cause increase blood pressure and/or pulse rate in some patients and may present a significant risk for patients with a history of coronary artery disease, congestive heart failure, arrhythmias, or stroke. Phenolphthalein was previously used in over-the-counter (OTC) laxatives, but was removed from the US market. Health risks linked to phenolphthalein could include potentially serious stomach upset, irregular heartbeat, and cancer with long-term use. Diclofenac is a nonsteroidal anti-inflammatory drug (NSAID) which could lead to serious side effects in patients taking other NSAIDs, with recent heart surgery, or with allergies to NSAIDs. NSAIDs can also lead to stomach bleeding, fatal heart attack or stroke.

In December 2015, the FDA recalled over a dozen products marketed by Bee Extremely Amazed LLC, of Jewett, OH. These products were also found to contain sibutramine and phenolphthalein, both products removed from the US market due to serious safety concerns, including heart side effects and cancer.

Many other 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 unknown ingredients.

See Also:

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U.S. Department of Health and Human Services. National Institutes of Health. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Prescription Medications to Treat Overweight and Obesity. Accessed October 21, 2016.

U.S. Food and Drug Administration. Questions and Answers: FDA Recommends Against the Continued Use of Meridia (sibutramine). Accessed October 21, 2016 at

Yanovski SZ, Yanovski JA. JAMA. Long-term drug treatment for obesity: A systematic and clinical review. 2014; 311(1):74–86

U.S. Food and Drug Administration. Beware of Fraudulent Weight-Loss 'Dietary Supplements'. Accessed October 21, 2016 at

FDA-Approved Weight Loss Drugs: Can They Help You? Accessed October 21, 2016 at