Orlistat is a pancreatic lipase inhibitor that blocks about 30 percent of dietary fat absorption. Xenical (120 mg) is the prescription strength; Alli (60 mg) is the OTC version. Average weight loss is 3 to 4 percent of body weight at one year. Side effects are mainly GI — oily stools, fecal urgency, flatulence — and a low-fat diet is essential to manage them.
What Orlistat Is
Orlistat is a chemically modified derivative of lipstatin, a natural lipase inhibitor. It was developed by Roche and FDA approved as Xenical in 1999 for chronic weight management. In 2007, the FDA approved a half-strength version (60 mg) sold over the counter as Alli (manufactured by GSK). Orlistat remains the only oral anti-obesity medication with a long FDA history that includes both prescription and OTC pathways.
Who Qualifies
Xenical (120 mg) is approved for adults with:
- BMI of 30 kg/m² or higher, or
- BMI of 27 kg/m² or higher with at least one weight-related risk factor
Xenical is also approved for adolescents 12 to 16 years old with BMI at the 95th percentile or higher for age and sex.
Alli (60 mg) is approved for adults 18 and older who are overweight (BMI 25 or higher) and want to lose weight with diet and exercise.
How It Works
Dietary fat must be broken down by pancreatic lipase into free fatty acids and monoglycerides before it can be absorbed across the intestinal wall. Orlistat binds to the active site of gastric and pancreatic lipases, blocking their function. With orlistat at the prescription 120 mg three-times-daily dose, roughly 30 percent of dietary triglyceride passes through the small intestine unhydrolyzed and is excreted in stool.
Caloric absorption is reduced by approximately 200 calories per day on a typical 2,000 kcal diet with 30 percent fat. Over months, this caloric deficit produces modest weight loss.
Doses
| Product | Strength | Dosing | Status |
|---|---|---|---|
| Xenical | 120 mg capsule | One capsule with each fat-containing meal, up to 3x daily | Rx |
| Alli | 60 mg capsule | One capsule with each fat-containing meal, up to 3x daily | OTC |
Orlistat must be taken during or up to one hour after a meal containing fat. Meals without fat (e.g., a plain salad with no dressing) do not require a dose — skip it. The drug is not absorbed systemically; less than 1 percent reaches the bloodstream.
Expected Weight Loss
| Trial / Setting | Mean Weight Loss at 1 Year | vs Placebo |
|---|---|---|
| Xenical 120 mg TID (Sjostrom 1998) | 10.2% | +3.9% vs placebo |
| Xenical 120 mg TID (Rossner 2000) | 9.7% | +3.4% |
| Alli 60 mg TID (Anderson 2006) | 5.3% | +2.0% vs placebo |
| XENDOS (4 years, prediabetes) | 5.8 kg (vs 3.0 kg placebo) | 37% lower T2D incidence |
The XENDOS trial randomized 3,305 obese patients (21 percent with impaired glucose tolerance — prediabetes) to orlistat or placebo plus lifestyle intervention for 4 years. Orlistat reduced cumulative incidence of type 2 diabetes from 9.0 percent to 6.2 percent (a 37 percent relative risk reduction), with the benefit concentrated in the prediabetes subgroup.
Common Side Effects
Almost all orlistat side effects are gastrointestinal and dose- and diet-dependent:
- Oily spotting on underwear — 27%
- Flatulence with discharge — 24%
- Fecal urgency — 22%
- Fatty or oily stool — 20%
- Oily evacuation — 12%
- Increased defecation — 11%
- Fecal incontinence — 8%
These side effects are directly proportional to dietary fat. A meal high in fat causes more pronounced effects; a low-fat meal causes few. Many patients experience the worst symptoms in the first few weeks and over time learn to eat lower-fat meals to avoid them — a behavioral feedback loop that itself supports weight loss.
Serious Risks
- Liver injury — rare but serious hepatotoxicity reported; the FDA added a label warning in 2010. Stop if signs of liver injury appear (jaundice, dark urine, abdominal pain).
- Acute kidney injury — from oxalate-related nephropathy; risk higher in patients with kidney disease.
- Pancreatitis — rare reports.
- Cholelithiasis — gallstones can be precipitated by rapid weight loss.
- Fat-soluble vitamin deficiency — A, D, E, K, and beta-carotene absorption is reduced. Daily multivitamin recommended, taken at bedtime or at least 2 hours separated from orlistat doses.
Drug Interactions
| Medication | Effect | Management |
|---|---|---|
| Cyclosporine | Reduced cyclosporine absorption | Avoid combination or take 3 hours apart; monitor levels |
| Levothyroxine | Reduced thyroxine absorption | Separate by 4 hours; monitor TSH |
| Warfarin | Vitamin K reduction may increase INR | Monitor INR closely |
| Antiepileptics (e.g., lamotrigine) | Possible decreased levels and seizures | Monitor levels and clinical status |
| Antiretrovirals | Reduced absorption reported | Monitor viral load |
| Amiodarone | Reduced absorption | Monitor |
| Multivitamins | Reduced fat-soluble vitamin absorption | Take at bedtime, separated from orlistat |
Contraindications
- Chronic malabsorption syndromes
- Cholestasis
- Pregnancy
- Known hypersensitivity to orlistat
Caution in patients with a history of oxalate kidney stones, hypothyroidism, eating disorders, and chronic GI disease.
How Orlistat Compares
| Option | Class | Avg Weight Loss (1 yr) | Cost |
|---|---|---|---|
| Orlistat (Xenical 120 mg) | Lipase inhibitor | ~6-10% | ~$700/mo brand, ~$200/mo generic |
| Orlistat (Alli 60 mg OTC) | Lipase inhibitor | ~5% | ~$50/mo |
| Phentermine | Sympathomimetic | ~5-8% (12 wk) | ~$10-30/mo |
| Qsymia | Phentermine + topiramate | ~8-10% | ~$98-200/mo |
| Contrave | Naltrexone + bupropion | ~5-9% | ~$99/mo DTC |
| Wegovy / Zepbound | GLP-1 / GIP-GLP-1 | ~15-21% | ~$1,000-1,350/mo |
Orlistat’s main niches today are OTC availability (Alli), proven prediabetes-to-diabetes prevention data, and use in patients who cannot or will not take centrally-acting drugs.
Practical Considerations
- Aim for less than 30 percent of calories from fat, distributed across meals.
- Take during or within 1 hour after a fat-containing meal.
- Skip the dose if a meal contains no fat.
- Take a daily multivitamin (A, D, E, K, beta-carotene) at bedtime.
- Carry an extra pair of underwear in the first weeks of treatment; oily spotting is unpredictable while you learn the diet.
- Track blood pressure, lipids, fasting glucose; in patients with prediabetes, document A1C trajectory.
- Levothyroxine, warfarin, and cyclosporine doses may need adjustment.
Related Reading
For broader context, see our anti-obesity medications overview, our Plenity guide, and our pillars on treatment options and whether prediabetes is reversible.
The Bottom Line
Orlistat (Xenical 120 mg Rx, Alli 60 mg OTC) is a pancreatic lipase inhibitor that blocks about 30 percent of dietary fat absorption, producing 3 to 4 percent weight loss above placebo at one year. The XENDOS trial showed it reduces progression from prediabetes to type 2 diabetes by 37 percent at four years. Side effects are predominantly GI — oily spotting, urgent stools, flatulence — and improve substantially with a low-fat diet. A daily multivitamin is essential, and orlistat interacts with cyclosporine, warfarin, and levothyroxine. Talk to your doctor about whether orlistat fits your goals and other medications.
