Since there's not really much research on the subject, I personally would stay away from it. Of course, I'm also the person who refuses to take a simple antihistamine while I'm nursing b/c I don't want it to affect my milk supply.

Also, some studies suggest that phen can be habit-forming, and if it does pass through your milk, you wouldn't really want to have to wean your daughter off of a diet pill - especially such a strong one, but that's just my $.02

You shouldn't be taking anything that could be potentially harmful to your baby, especially something that isn't necessary for you at this point. If you want to diet, consider waiting until after the baby has stopped nursing.

The other factor to consider is that if you are dieting or taking diet pills while nursing is that if you lose weight too quickly or unnaturally, too many fat cells will be burned at once. Our fat cells retain toxins and once these fat cells are burned, the toxins are released into our system and end up in the breast milk. Those toxins, along with the diet pill could make things really hard on your baby.