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Orlistat side effects: what to expect

yael coopermandanielle oaks

Reviewed by Yael Cooperman, MD, written by Danielle Oaks

Last updated: Mar 19, 2021
4 min read


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

Orlistat, also found under brand names Xenical or Alli, is a weight loss medication used to treat obesity. 

It’s effective when taken with meals for several months, but it’s not without side effects, which tend to affect the digestive system. People typically experience side effects right when treatment starts, and often feel better within a month (Jain, 2011).


weight loss

Lose 15% of your body weight in one year on average


weight loss

Lose 15% of your body weight in one year on average

What are the most common side effects of orlistat? 

Both prescription and over-the-counter orlistat can be effective in helping you lose weight and keep it off, but it’s important to be aware of the side effects it could cause. They may be unpleasant, but for most people, they don’t last long. 

Orlistat works by reducing how much fat your body absorbs from your food, leaving excess in your digestive system to be released as waste. But since it increases the amount of fat that travels through your digestive system, it can cause some uncomfortable side effects when you eat meals that are high in fat.

Sticking to a low-fat diet while taking orlistat can lower your chance of experiencing any uncomfortable side effects.

While absorbing less fat is a good way to lose weight, fat isn’t necessarily a bad thing. Four of the most important vitamins we need to stay healthy, vitamins A, D, E, and K, need fat to be absorbed. Because this drug changes the way your body absorbs fat, it can also interrupt the way your body absorbs those important fat-soluble vitamins.  

While taking orlistat, it’s recommended that you eat a low-calorie diet, and try to spread the amount of fat, protein, and carbohydrates you eat each day evenly over three meals.

Here are some side effects people most frequently see when taking this weight loss drug:

  • Passing gas, sometimes with oily spotting
  • Loose stools, greasy stools, or diarrhea
  • Frequent stools or bowel movements that are hard to control
  • Constipation
  • Abdominal pain
  • Bloating

Aside from the digestive complaints, headache is the next most reported side effect.  Any type of reaction to orlistat usually occurs within the first three months of taking it, and about half the time side effects will disappear within a week. Most go away within a month but can remain for longer (Rizo Treviño, 2018).

Does orlistat have any serious side effects?

Rarely, people taking orlistat have experienced severe liver problems. Though the U.S. Food and Drug Administration (FDA) has added educational information about liver injuries to Xenical and Alli packaging, the evidence isn’t strong enough to definitively say that orlistat causes liver damage (FDA, 2018). 

Still, you should let a healthcare provider know if you notice any signs of liver problems while taking orlistat. Symptoms to watch out for include itching, yellow eyes or skin, dark urine, loss of appetite, or light-colored stools.

If you notice adverse effects or have any concerns, the best place to start is talking to a healthcare provider. If you have severe or constant abdominal pain, stop using orlistat right away.

What else should I know about taking orlistat?

Orlistat is a weight-loss medication available in two FDA-approved dosages: prescription Xenical in 120 mg and over-the-counter Alli in 60 mg (FDA, 2015).

It’s approved for use in patients with a body mass index (BMI) of 30 and up or those with a BMI of 27 and higher coupled with a condition like diabetes, high blood pressure, or high cholesterol.

It’s also recommended that you take a daily multivitamin containing vitamins A, D, E, and K, as well as beta-carotene while using orlistat. Do not take these multivitamins at the same time as orlistat. It’s best to take a daily multivitamin at least two hours after taking orlistat so your body can absorb it properly. Orlistat should be taken during a meal or within an hour of eating (Bansal, 2020).

Orlistat can also affect the way other drugs work in the body, so make sure to let your  healthcare provider know about any other medications or supplements you’re taking. Here are some medications known to interact with orlistat: 

  • Cyclosporine 
  • Anticoagulants (blood thinners, like warfarin)
  • Amiodarone (heart medication) 
  • Medications for seizures
  • Antiretroviral medications like those used to treat HIV 
  • Diabetes medications
  • Thyroid medications like levothyroxine
  • Other weight loss medications

If you’re taking any of these, your healthcare provider might advise you not to use orlistat or give you guidance about when it’s safe to take orlistat to avoid drug interactions.

Children, people who are pregnant or breastfeeding, and people with a history of eating disorders like anorexia or bulimia should avoid using orlistat.


  1. Bansal, A. B., Al Khalili, Y. (2020, November 22). Orlistat. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK542202/#article-26335.s5
  2. U.S. Food and Drug Administration. (2015, July). Orlistat (marketed as Alli and Xenical) Information. Retrieved March 2, 2021 from https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/orlistat-marketed-alli-and-xenical-information
  3. U.S. Food and Drug Administration. (2018, February 6). Completed safety review of Xenical/Alli and severe liver injury. Retrieved March 2, 2021 from https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/fda-drug-safety-communication-completed-safety-review-xenicalalli-orlistat-and-severe-liver-injury
  4. Genentech, Inc. (2015, August 1). Xenical (orlistat): Highlights of Prescribing Information. https://www.accessdata.fda.gov/drugsatfda_docs/label/2015/020766s035lbl.pdf
  5. GSK Consumer Healthcare. (2017, May 15). Alli Orlistat label. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/021887Orig1s009lbl.pdf
  6. Jain, S. S., Ramanand, S. J., Ramanand, J. B., Akat, P. B., Patwardhan, M. H., & Joshi, S. R. (2011). Evaluation of efficacy and safety of orlistat in obese patients. Indian Journal of Endocrinology and Metabolism, 15(2), 99–104. https://pubmed.ncbi.nlm.nih.gov/21731866/
  7. Rizo Treviño S. (2018). Demographic and clinical characteristics, and adverse reactions of people with overweight and obesity consumers of orlistat, attended by a call center (2009-2017). Características demográficas, clínicas y reacciones adversas de personas con sobrepeso y obesidad: consumidores de orlistat atendidos por centro de atención telefónica (2009-2017). Medwave, 17(6), e7288. https://pubmed.ncbi.nlm.nih.gov/30507894/
  8. U.S. Food and Drug Administration, Adverse Event Reporting System (FAERS) Public Dashboard. (2020, December 31). ORLISTAT. Retrieved March 2, 2021 from https://fis.fda.gov/sense/app/d10be6bb-494e-4cd2-82e4-0135608ddc13/sheet/59a37af8-d2bb-4dee-90bf-6620b1d5542f/state/analysis

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.