Randomised controlled trials
In the trials below, people were assigned at random to take Orlistat or a dummy pill (placebo). All the weight loss figures are averages: how much body weight fell from the start.
| Trial | Key result | Source |
|---|---|---|
| [1]XENDOS | 2.8 kg more weight loss than placebo at 4 years. 37% reduction in progression to type 2 diabetes vs. placebo plus lifestyle intervention. | Diabetes Care 2004 ↗ |
| [2]1-year randomised controlled trial (Sjöström et al.) | 8.5% average weight loss in the orlistat group vs 5.4% placebo — the most commonly cited 1-year efficacy figure. The drug's effect is modest compared to GLP-1 class drugs but it works through a completely different mechanism (blocking fat absorption in the gut, not the brain). | International Journal of Obesity 2000 ↗ |
Reported side effects
The rates below come from the XENDOS trial. View source
| Side effect | Frequency in trial |
|---|---|
| Oily/fatty stools (steatorrhoea) | 27% |
| Faecal urgency | 22% |
| Oily spotting on underwear | 27% |
| Faecal incontinence | 8% |
| Abdominal pain/discomfort | 14% |
Body composition data
No specific body composition data distinguishes orlistat from calorie-restriction alone. The drug produces weight loss entirely through calorie malabsorption — lean mass effects depend on the degree of calorie deficit and protein intake.
Heart and metabolic data
In the 4-year XENDOS trial, orlistat reduced progression from prediabetes to type 2 diabetes by 37% compared to placebo plus lifestyle changes. Modest improvements in blood pressure and LDL observed.