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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Anti-obesity drugs

Drugs used with diet and activity to lose weight. This page covers orlistat, which blocks fat absorption in the gut.

Updated 2026-11-01

## What they are

Peripherally acting drugs (orlistat) and centrally acting ones. GLP-1 agonists such as semaglutide, liraglutide and tirzepatide are also used for obesity and have their own pages.

## Mechanism of action

Orlistat forms a slowly reversible covalent bond with the active-site serine of gastric and pancreatic lipases in the gut lumen. The enzymes can no longer split dietary triglycerides into absorbable free fatty acids and monoglycerides, so part of the fat eaten passes out in the stool. It acts in the gut and is barely absorbed.

Target: Gastric and pancreatic lipases (orlistat)

## Key points

- Orlistat side effects are oily spotting, flatus with discharge and urgency, worse with fatty meals.
- It lowers absorption of fat-soluble vitamins (A, D, E, K), so a multivitamin is given apart from it.
- Contraindicated in pregnancy, chronic malabsorption and cholestasis.
- Drugs are an add-on to lifestyle change, never a replacement.

## Drugs in this class

- [**Orlistat** An anti-obesity drug that blocks fat-digesting enzymes in the gut, so part of the fat eaten leaves in the stool. Pancreatic and gastric lipases](https://durgical.com/en/pharmacology/orlistat/)

## Others in the class

- phentermine–topiramate
- naltrexone–bupropion
- setmelanotide (MC4R agonist, rare genetic obesity)

## Test yourself

Think of the answer, then tap the question to see it.

## Why does orlistat cause oily stools?

Undigested triglycerides, which lipases could not split, stay in the gut and leave with the stool.

## Which vitamins can fall with long-term orlistat?

The fat-soluble vitamins A, D, E and K (and beta-carotene).

## Where does orlistat act?

In the lumen of the stomach and small intestine, on gastric and pancreatic lipases; very little reaches the blood.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Orlistat: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a601244.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [DailyMed - ORLISTAT capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f81dfaeb-46d5-47ce-9ef6-19259f5ac61c) (DailyMed (U.S. National Library of Medicine), 2026).

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