Alli Weight-Loss Pills: Short-Term and Serious Side Effects to Know About

If you've ever browsed the aisles at your local pharmacy, you may have seen Alli (orlistat) on the shelves. It’s a pill that can help with weight loss. So if diet and exercise alone haven’t helped you reach your weight-loss goal, you might consider adding Alli to your regimen.

After Xenical (the prescription-only version of Alli) was approved in 1999, news spread about its unique side effects. Alli was later approved as the over-the-counter (OTC) option, but potential safety concerns came to light. The FDA has since completed a thorough safety review and updated the medication’s packaging label.

Let’s take a look at the common and serious side effects of Alli, how to manage them, and when to talk to your healthcare provider.

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What is Alli (orlistat)?

Alli is an OTC medication that you can get without a prescription. It also has a higher-dose prescription version called Xenical.

Alli is FDA approved for weight loss for adults 18 or older who are considered overweight. It works best when combined with a reduced calorie, low-fat diet and regular exercise. Alli is available as a capsule that you take by mouth with fat-containing meals, up to 3 times a day.

How does Alli work for weight loss?

When you eat, your stomach and pancreas release lipases. Lipases break down the fat in the food you eat. This allows your body to absorb and store the fat.

Alli stops these lipases from working. This prevents your body from digesting and storing some of the fat you eat. Fat that isn’t absorbed is passed through your body and eliminated in your bowel movements.

When taken in combination with dietary changes, Alli can help provide some additional weight loss. So, if you were to lose 5 lbs from diet alone, Alli could provide an additional 2 lbs to 3 lbs of weight loss.

Common Alli side effects

Alli has little effect on your entire body. It stays in your stomach and intestines. While taking Alli, your body will eliminate extra fat instead of storing it. Fat traveling through your digestive system can cause unpleasant side effects.

Examples of Alli side effects include:

• Loose stools

• Oily stools

• More frequent bowel movements

• Gas with oily spotting

• Stomach pain

How long do Alli side effects last?

Most of the common Alli side effects improve after a couple of weeks. These side effects are mostly related to diarrhea and loose, oily stools as your body is getting rid of extra fat.

How to manage Alli side effects

Replacing high-fat foods with lower-fat alternatives can help reduce these unwanted Alli effects. You may also want to stay close to a bathroom until you see how your body reacts to starting Alli.

Common high-fat foods that may worsen Alli side effects include:

• Butter

• Cheese

• Desserts

• Whole eggs

• Fried fish

• Bacon

If you’re gassy, OTC options might help. These include products that contain simethicone (e.g., Gas-X). Because of how Alli works, antidiarrheals like loperamide (Imodium A-D) generally won’t help provide diarrhea relief.

Serious Alli side effects

Although Alli is an OTC medication, there are possible serious side effects that you should be aware of.

Reduced vitamin absorption

Alli doesn’t just block the absorption of fat. It can also lessen the absorption of fat-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. These vitamins must mix with the fat in your diet to be absorbed and stored by the body. Since Alli lessens fat absorption, fat-soluble vitamins are also affected.

Fat-soluble vitamins have many important jobs in the body. To ensure that your levels don’t drop too low, it’s recommended to take a daily multivitamin with fat-soluble vitamins at bedtime. Your healthcare provider or pharmacist can recommend options that best meet your needs.

Liver problems

Severe liver injury has been reported with Alli. However, the FDA extensively reviewed these reports and concluded that severe liver damage is rare.

When looking at an estimated 40 million people who took Alli or Xenical, the FDA found 13 cases of severe liver injury. One of these cases was associated with Alli and the rest with Xenical. Researchers could not establish a “cause-and-effect” relationship as other medications and conditions may have been contributing factors.

It’s still important for you to watch for potential symptoms of liver injury and contact your healthcare provider if you notice any. These include loss of appetite, light-colored stool, or yellowing of your skin or eyes (jaundice). Untreated liver injuries may require a liver transplant or even lead to death.

Kidney problems

The fat that’s not absorbed by your body can bind with calcium and lead to deposits in your kidneys. This can potentially lead to kidney stones and acute kidney injury. Contact your healthcare provider if you start experiencing lower back or side pain, nausea, and/or the inability to urinate. Alli shouldn’t be used if you have severe kidney problems.

Gallstones

Weight loss, with or without Alli, can raise your risk of gallstones. Contact your healthcare provider if you feel pain in the upper-right area of your stomach. Nausea and vomiting may also accompany stomach pain.

Pancreatitis

Although rare, pancreatitis (swelling of the pancreas) has been reported in people taking Alli. Early symptoms can include nausea, vomiting, and severe upper-right abdominal pain. This can progress to fever, low blood pressure, and shortness of breath. If these symptoms develop, go to the nearest emergency room. If left untreated, pancreatitis can be life-threatening.

When should you talk to your healthcare provider about Alli side effects?

Many common Alli side effects can be managed at home and should go away after a few weeks of treatment. A few changes to your diet, such as choosing low-fat foods, can help in the meantime. But if any side effects aren’t going away or are getting worse, contact your healthcare provider.

Symptoms such as jaundice and severe abdominal pain are more serious and require immediate medical attention. Go to the nearest ER if these symptoms, or any others that feel severe or life-threatening, develop.

Good to know: Alli may also affect the absorption of other medications and supplements. If you’re taking other medications, it’s important to make sure Alli is a safe option. Make sure your provider and pharmacist have a full list of your medications and supplements to check for interactions.

Is Alli safe for you?

It depends. Alli may not be safe for everyone. For example, people who have difficulty absorbing nutrients from food or cholestasis (slowed or blocked bile flow) shouldn’t take Alli. You also shouldn’t take Alli if you’re pregnant. This is because weight loss during pregnancy can be harmful to a growing fetus.

Even though it’s available OTC, it’s important to talk to your provider before starting Alli. Since Alli can interfere with other medications you’re taking, they may need to adjust your dosage or monitor you more closely. They can also review your medical history to ensure Alli is safe for you to take.

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Written by:

Brian Leonard, PharmD, BCACP, BCGP

Brian Leonard earned his doctorate in pharmacy from the University of Florida College of Pharmacy and is board certified in Ambulatory Care and Geriatric Pharmacy.

Edited by:

Alyssa Billingsley, PharmD

Alyssa Billingsley, PharmD, is the director of pharmacy content for GoodRx. She has over a decade of experience as a pharmacist and has worked in clinical, academic, and administrative roles.

Reviewed by:

Christina Aungst, PharmD

Christina Aungst, PharmD, is a pharmacy editor for GoodRx. She began writing for GoodRx Health in 2019, transitioning from freelance writer to editor in 2021.

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