Is Alli the same as the prescription drug?

Same active ingredient (orlistat), but half the dose. Alli is 60 mg; prescription Xenical is 120 mg. The mechanism is identical but the OTC dose blocks less fat — roughly 25% compared to about 30% for the prescription strength.

Do the side effects go away?

Yes, usually — and they are entirely within your control. The oily stools and urgency are caused by unabsorbed fat passing through. Keeping each meal under about 15g of fat virtually eliminates them. Most people work out their fat limit within the first few weeks.

Do I need to take a vitamin?

Yes. Orlistat blocks fat-soluble vitamins (A, D, E, K) as well as fat. Take a daily multivitamin at bedtime — at least 2 hours after your last orlistat dose — to ensure proper absorption.

Can I take it without any diet changes?

Technically yes, but you won't get the full benefit. The drug works best when combined with a reduced-fat diet (under 30% of calories from fat). Eating high-fat meals while taking Alli causes significant GI side effects, which naturally discourages high-fat eating.

Stronglipase inhibitor

Orlistat OTC (Alli)

The only FDA-approved over-the-counter weight loss drug. A half-strength version of prescription Xenical (60 mg vs. 120 mg). Works by blocking approximately 25% of dietary fat from being absorbed in the intestine.

Quick read · 3 min

Estimated cost

~$40–$60 for a 60-day supply (OTC, no prescription needed)

How to take

Three capsules daily with each fat-containing meal — requires consistent timing around meals

StrongLarge, well-conducted trials

In clinical trials of prescription-strength orlistat (120 mg, Xenical), participants lost approximately 2.9 kg more than placebo over one year in a meta-analysis of 17 randomised controlled trials.

How it works

Blocks pancreatic lipase enzymes in the small intestine, preventing about 25–30% of dietary fat from being broken down and absorbed. The undigested fat passes through the digestive system and is excreted. The GI side effects are directly tied to fat intake — eating a high-fat meal worsens them significantly.

What the evidence shows

Strong

In clinical trials of prescription-strength orlistat (120 mg, Xenical), participants lost approximately 2.9 kg more than placebo over one year in a meta-analysis of 17 randomised controlled trials. The 4-year XENDOS trial showed 2.8 kg additional weight loss versus placebo plus lifestyle changes, and a 37% reduction in progression to type 2 diabetes. The OTC dose (60 mg) has less evidence than the prescription strength but uses the same mechanism at half the dose.

The trade-off

What this tends to offer:

✓FDA-approved with long-term safety data (since 1999)
✓No prescription needed (OTC version)
✓Diabetes prevention benefit in XENDOS trial
✓Cost: ~$40–$60/month — among the most affordable options

What this involves:

•GI side effects (oily stools, urgency) if fat intake is high
•Blocks fat-soluble vitamins (A, D, E, K) — daily multivitamin needed
•Three capsules daily with meals — consistent timing required
•Modest weight loss compared to prescription medications

Safety

The main side effects are GI-related: oily or greasy stools, faecal urgency, and potential faecal incontinence — particularly if a high-fat meal is eaten. These are not dangerous but can be embarrassing and inconvenient. Orlistat also reduces absorption of fat-soluble vitamins (A, D, E, K) — a multivitamin taken at bedtime (away from doses) is recommended. Rare cases of severe liver injury have been reported, though the link is uncertain.

Community insights

These are personal experiences shared in public online communities — not medical advice.

“The side effects are the mechanism — they 'punish' high-fat eating. Many people accidentally adopt a low-fat diet because they want to avoid the GI consequences.”

r/loseit·Commonly explained to people new to Alli

“Keep dark clothing and liners on hand for the first few weeks. Most people say the side effects improve once they learn the right fat intake level.”

r/WeightLossAdvice·Practical advice from experienced users

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Last reviewed: June 2026

Medical disclaimer: This website is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any treatment.