Phentermine is generic, the least expensive of the medications, and has been on the market the longest. Once a part of the famed fen-phen combination weight loss drug, Phentermine works to decrease appetite.

It is the most widely used weight loss drug. While it's officially approved only as a short-term drug, some doctors use it as long term therapy.

"I use phentermine for people with strong appetites. [It] seems to work best on reducing hunger and the number of eating episodes. I also like it because it is generic, and inexpensive for patients who do not have insurance coverage," says Kushner.

Side effects may include depression, insomnia, increased blood pressure, irritability, and nervousness.