Chitosan is a fiber made from the shells of crustaceans, sold as a "fat blocker" that binds some dietary fat in the gut. A Cochrane review found that its effect on body weight in high-quality trials is minimal and unlikely to matter clinically.
Also known as: deacetylated chitin.
Trials ODS describes used 2 to 3 g a day, split before meals, for 28 days to 8 weeks; trials in the Cochrane review lasted 4 weeks to 6 months (ODS). These are study protocols, not a recommendation.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. How it differs from GLP-1 medicines: semaglutide (Wegovy) and tirzepatide (Zepbound) are prescription drugs whose FDA labels include reducing excess body weight and keeping it off long term in adults with obesity, or with overweight and a weight-related condition, alongside diet and exercise. Chitosan is not an FDA-approved drug for weight loss.
What studies show: a Cochrane review of 15 trials (1,219 people) found about 1.7 kg more weight loss than placebo, but many trials were of poor quality, and larger, longer and better-designed trials showed much smaller effects; the authors concluded the effect on weight is minimal and unlikely to be of clinical significance (Jull 2008). In a 12-day fat-absorption study, 2.5 g a day slightly increased fat in stool in men, an amount ODS says would remove only about 1 lb of body fat over about 7 months, and had no effect in women.
Safety: side effects are mainly in the gut: flatulence, bloating, mild nausea, constipation, indigestion and heartburn (ODS). Because it is made from shellfish, people allergic to shellfish could in theory react to it (ODS). Chitosan might strengthen the blood-thinning effect of warfarin (ODS, Huang 2007). FDA warns that many products sold for weight loss, including supplements and teas, are likely to contain dangerous hidden drug ingredients, and that its public list of such products covers only a small fraction of them.
Chitosan is meant to act in the gut by binding some dietary fat (ODS), and we could not find a published half-life or time to peak, so we do not list one.
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