Obesity is epidemic and many obese patients seek drug treatment. Orlistat (a lipase inhibitor) is effective, but associated with gastrointestinal side effects. This study aimed to demonstrate that Cetilistat inhibits gastrointestinal lipase and is well tolerated in comparison to Orlistat. Results from three Phase I clinical studies (all double-blind, randomized, placebo-controlled, parallel-group studies in healthy male volunteers (age 18-45y, BMI ≤30 kg m−2, minimum weight of 60 kg) (total n=99) resident in a clinical unit) are reported. Subjects receives placebo (n=24), one of a range of Cetilistat doses (n=66) or 120mg Orlistat x3/daily (n=9) for 5 days, whilst maintained on a strict diet. 30% of calories were derived from fat. Each dose of Cetilistat in the 3 studies increased mean faecal fat excretion relative to both baseline and placebo. Increased faecal fat excretion was significant compared with placebo at 150 mg t.i.d. (study 1). Faecal fat excretion was significantly higher in both the Cetilistat 120 and 240 mg t.i.d. groups (study 3). Overall, fecal fat excretion for Orlistat 120 mg t.i.d. was similar to Cetilistat. Gastroinetstinal side effects were most common (51%) for Cetilistat. Steatorrhoea (oily stool) was more frequent in the Orlistat group (4.11 events per subject) than in any Cetilistat dose group. Most side-effects (98%) were mild or moderate. 3 subjects taking Cetilistat were withdrawn from the study after developing maculopapular rash, but no reaction was observed (pin-prick). The chemical structure of Orlistat and Cetistat are different which may affect how they interact with fat micelles in the intestines. The author speculates that Cetilistat may act more like a detergent, whereas Orlistat may promote the coalescence of micelles, leading to oils and increased gastrointestinal adverse events. Cetilistat is an effective inhibitor of gastrointestinal lipases, increasing faecal fat excretion in at all doses studied. Cetilistat is well tolerated across a range of doses, and comparison with Orlistat suggests improved tolerability.