Orlistat: what it treats, how to take it, side effects
Last updated September 3, 2026.
Orlistat (prescription Xenical 120 mg, over-the-counter Alli 60 mg) is a weight-loss pill that works in your gut, not your brain: it blocks about 30% of dietary fat from being absorbed. That fat has to go somewhere, which produces the most talked-about side effects in the weight-loss world, and a built-in incentive to eat less fat.
What does it treat?
Obesity and overweight, alongside a reduced-calorie, low-fat diet (about 30% of calories from fat), for losing weight and for keeping lost weight from returning. The prescription version is used in people with related conditions like diabetes, hypertension, or high cholesterol. It is not a stimulant and has no abuse potential, which distinguishes it from most older weight drugs.
How do you take it?
One 120 mg capsule (prescription) three times a day with each main meal containing fat, during the meal or up to 1 hour after. Skip the dose if you skip the meal or the meal has no fat. The label is clear that doses above 120 mg three times daily add nothing. Because it blocks fat-soluble vitamins (A, D, E, K) and beta-carotene, take a daily multivitamin at least 2 hours before or after orlistat, bedtime is the classic slot. Cyclosporine must be separated by 4 hours. Missed dose: take it if within an hour of the meal; otherwise skip.
Side effects to know
- Common: the unabsorbed-fat collection: oily spotting, gas with discharge, fecal urgency, fatty or oily stools, and increased defecation. They are worst in the first weeks and shrink dramatically when meals actually stay low-fat, which is the behavior modification hiding inside the side effect.
- Serious: rare severe liver injury (watch for itching, yellow eyes, dark urine), kidney stones from oxalate, and gallstones with rapid weight loss.
Who should not take it?
Not with chronic malabsorption syndrome or cholestasis, and not in pregnancy. It interacts meaningfully: cyclosporine (separate by 4 hours), warfarin (INR monitoring, because vitamin K absorption shifts), levothyroxine (separate by 4 hours and monitor), and some seizure and HIV drugs. The OTC version (Alli) carries the same rules at half strength.
When is it an emergency?
Call 911 for signs of severe liver injury with confusion or bleeding, or for severe abdominal pain that could signal gallstone complications. Yellowing skin or eyes, dark urine, or pale stools need same-day contact with your doctor. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
How much weight do people lose on orlistat?
In trials, about 5 to 8% of body weight over a year versus 2 to 3% on placebo, with diabetes-prevention benefits as a bonus. It is modest, but it is also the only major weight pill with no systemic absorption to speak of, which is its entire appeal for people avoiding stimulants or hormones.
Are the GI side effects as bad as people say?
They are as bad as your diet is fatty. The mechanism is mechanical: blocked fat stays in the gut and exits as oily spotting and urgency. People who keep meals near 30% fat report mild or no effects after the first weeks; people who eat fried food on orlistat learn once. It is the only drug whose side effect is a diet coach.
What is the difference between Xenical and Alli?
Same drug, different dose and status. Xenical is 120 mg prescription, three times daily with meals. Alli is 60 mg over the counter, same schedule. Alli is about 25% less effective per the math and carries the same rules: low-fat diet, mealtime dosing, and the multivitamin gap.
Why do I need a multivitamin with it?
Orlistat blocks absorption of fat-soluble vitamins A, D, E, K and beta-carotene along with the fat. A daily multivitamin taken at least 2 hours apart, bedtime is easiest, covers the gap. Long-term use without it can produce real deficiencies, and warfarin patients need INR monitoring because vitamin K shifts.
Can I skip the pill when I eat a fat-free meal?
Yes, and the label says so directly: no fat in the meal, no point in the dose. Missed the mealtime window by more than an hour? Skip that dose entirely. It is one of the few medications where skipping is sometimes the correct move.
Is orlistat safe long-term?
Years-long data exist and the systemic exposure is minimal. The flagged long-term issues are the vitamin deficiency (managed with the multivitamin), rare oxalate kidney stones, and rare severe liver injury, which is why itching plus dark urine is a stop-and-call combination. Regular follow-ups keep the accounting honest.