What to Expect When Starting Orlistat
Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting, stopping, or changing any medication. Using this site does not create a doctor-patient relationship.
Drug information changes as the FDA updates labeling, and we cannot guarantee it is complete or current. Verify critical details with your pharmacist or physician.
Emergencies: If you think you may have a medical emergency, call 911 immediately. For a suspected overdose, call Poison Control at 1-800-222-1222. Report side effects to the FDA MedWatch program at fda.gov/medwatch or 1-800-FDA-1088.
See our Terms of Use and Editorial Policy.
Introduction
Orlistat (Xenical, Alli) is a lipase inhibitor that blocks the absorption of approximately one-third of dietary fat, reducing caloric intake and promoting weight loss. It is taken with each fat-containing meal, up to three times daily. The gastrointestinal side effects — oily spotting, loose stools, and urgent bowel movements — are directly related to the amount of fat consumed and serve as a behavioral feedback mechanism to limit dietary fat.
Week-by-week timeline
Starting Orlistat
Take one 120 mg capsule (Xenical) or 60 mg capsule (Alli) with each main meal containing fat. If a meal has no fat, skip that dose. Expect GI side effects: oily or fatty stools, increased bowel movements, and possible urgency. These are most pronounced early on and when high-fat meals are eaten.
Adapting Dietary Habits
Most patients quickly learn to limit fat intake to avoid uncomfortable GI effects. Target fat intake of 15-20 grams per meal. The discomfort becomes a motivator for dietary compliance. Weight loss of 1-2 pounds per week is typical with caloric restriction.
Establishing Patterns
GI side effects become less frequent as dietary habits stabilize around lower fat intake. Consistent weight loss continues. Take a daily multivitamin at bedtime (2 hours after orlistat) as fat-soluble vitamins A, D, E, K absorption is reduced.
Long-Term Results
Clinical trials show average weight loss of 5-7% of body weight after 6-12 months compared to 2-3% with diet alone. Orlistat is approved for up to 4 years of use. Blood pressure, blood sugar, and cholesterol improve alongside weight loss.
When to call your doctor
Contact your healthcare provider if you experience:
- Signs of severe liver injury: yellowing of skin or eyes, dark urine, loss of appetite, light-colored stools
- Severe or unusual stomach pain
- Kidney stones or severe kidney pain (rare association with calcium oxalate stones)
- Rectal bleeding
- Difficulty controlling bowel movements that is severe or worsening
- Signs of allergic reaction: hives, difficulty breathing
Tips for getting started
Spread fat intake evenly across meals — no more than 15-20 grams of fat per meal. High-fat meals will cause significant GI discomfort, which is a direct feedback signal. Wear dark-colored underwear in the initial weeks while you adjust your diet. Take a multivitamin supplement containing fat-soluble vitamins (A, D, E, K) at bedtime, separated from orlistat doses. Combine orlistat with a calorie-reduced diet and regular exercise for best results.
Frequently asked questions
More about Orlistat
References
- [Regulatory] FDA Prescribing Information for Xenical (orlistat) https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/020766s033lbl.pdf Accessed 2026-03-01.
- [Regulatory] NIH MedlinePlus: Orlistat https://medlineplus.gov/druginfo/meds/a601244.html Accessed 2026-03-01.
- [Clinical] Cochrane Review: Orlistat for Obesity https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD003641.pub4 Accessed 2026-03-01.
Written and fact-checked by PrescriptionDrugs.org Editorial Team
Last updated: