How Alli Works to Help You Lose Weight

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Alli is an over-the-counter (OTC) diet pill approved by the U.S. Food and Drug Administration (FDA). It is the brand name for a medication called orlistat, which is perhaps better known by its prescription version's name: Xenical. Alli is FDA-approved for use in overweight adults along with exercise and a reduced-calorie, low-fat diet.

This diet pill is not an easy fix for weight loss. But it may help some people lose more weight than if they used lifestyle changes alone. In turn, losing weight can improve health.

How Alli Works

Alli is a lipase inhibitor. It works by reducing the absorption of fat in the body. Alli's manufacturer suggests that taking the medication three times a day (one hour before, during, or after each meal) will remove about 25% of the fat you consume from your intestines. Excess fat is eliminated in the stool.

Taking Alli doesn’t mean you can indulge in extra trans fats and other unhealthy foods. The medication should be used in conjunction with a well-balanced, low-calorie, low-fat diet. It is also important that you consume healthy fats. You should aim for 15 grams of fat per meal, which means that you’ll eat less than 30% of your total daily calories from fat.

Alli contains 60 mg of orlistat. Xenical contains 120 mg of the drug and requires a prescription from your healthcare provider.


How much weight you can lose when you take Alli depends on a number of factors, including recommended lifestyle changes. If you don't adhere to the recommended healthy diet and lifestyle, you probably won’t lose weight. And if you don’t also exercise on a regular basis, any weight you do lose probably won’t stay off for good.

Results of research on weight loss with orlistat have been mixed. A 2007 review found that participants who took a 60 mg dose of orlistat lost an average of 5% of their initial total body weight in four months.

Another study published in 2015 tracked over 100,000 orlistat users over a three-year period. Researchers found that although patients lost just shy of a pound and a half per month during the first few months, they did not maintain significant weight loss for the long term.

Side Effects

The most common side effects of Alli involve changes to bowel movements. These can include gas and loose or oily stools, stomach cramps, oily spotting on underwear, and difficulty controlling bowel movements.

Many Alli users who complain about side effects experience them in response to eating a high-fat meal. But even with a careful diet, some users can have problems.

Side effects generally subside after the first few weeks of taking the pill. If Alli side effects continue beyond that point, talk to your doctor.

Risks and Warnings

In addition to side effects, Alli may be harmful to the liver. In 2010, the FDA updated its requirements for the labeling of orlistat products to include safety information about a few cases of severe liver injury linked to the weight-loss pill. If you have a pre-existing liver condition or experience jaundice, severe itching, or dark urine while taking Alli or Xenical, see your doctor.

As with all medications, purchase Alli from a reputable vendor and check the package closely before taking a pill. In 2014, product tampering resulted in a voluntary recall of the diet pills. Alli came back on the market in 2015 with updated tamper-resistant packaging .

Is Alli Right for You?

If you are thinking about taking Alli for weight loss, talk to your doctor. While a prescription is not required, you should not take orlistat if you have any of the following conditions:

  • Pregnancy
  • Cholestasis
  • Malabsorption syndrome
  • Anorexia nervosa
  • Bulimia
  • Diabetes
  • Kidney stones
  • Pancreatitis
  • Gallbladder disease
  • Thyroid disease

Your doctor is the best person to determine if the product is safe for you and how losing weight might improve your health.

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8 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  4. Douglas IJ, Bhaskaran K, Batterham RL, Smeeth L. The effectiveness of pharmaceutical interventions for obesity: Weight loss with orlistat and sibutramine in a United Kingdom population-based cohort. Br J Clin Pharmacol. 2015;79(6):1020-1027. doi:10.1111/bcp.12578

  5. U.S. National Library of Medicine. Orlistat. Updated July 21, 2020.

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  8. Hirschler B. GSK recalls weight-loss drug Alli in U.S. on tampering concerns. Reuters, 2014.