How quickly can orlistat make you lose weight?

Modest weight loss. Prescription strength people lost 6 pounds over a year and over the counter version people lost 3 pounds over a year per review articles.
It depends. Diet pills work better when combined with lifestyle modification including proper nutrition and exercise. With orlistat (alli, xenical) you can expect 4 pounds or more per month. The side effects of orlistat including oily stains, fatty stools and fecal incontinence make it less desirable as a diet pill.