Over the counter weight loss pills generally contain some sort of stimulant, sometimes caffeine. Frequently cause side effects such as nausea, palpitations, anxiety. Can be dangerous to take for people with heart conditions. I generally recommend against these.
The two prescription weight loss agents recommended by the FDA are Meridia (sibutramine) and Xenical (orlistat), which is soon to be sold without a prescription under the name Alli. I'm not against either of these pills, however, I have yet to prescribe them once. Once I tell patients that:
1. Insurance often doesn't cover.
2. Weight loss at one year averages 15-20 pounds (compared to 8-10 for placebo), and decreases thereafter.
3. Xenical's list of common side effects include excessive gas, oily discharge, and fecal incontinence.
No one will bite. I can't say I blame them. If there was a weight loss pill that could get you 30-40 pounds, I think people would go for it. But most people weighing 300 pounds can't get real excited about 287, when it means taking an expensive daily pill with side effects.
So it's not that I won't prescribe diet pills, but I don't.