Orlistat side effects: what to expect

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).

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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).

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.

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.

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Current version

March 19, 2021

Written by

Danielle Oaks

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

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