For what it's worth, my surgeon is recommending I get an echocardiogram prior to surgery because I took fen-phen back in the late 90s for a little while.  I wouldn't recommend taking phentermine at all... per wikipedia:

 

In 1959 phentermine first received approval from the FDA as an appetite suppressing drug. Phentermine hydrochloride then became available in the early 1970s. It was previously sold as Fastin from King Pharmaceuticals for SmithKline Beecham, however in 1998 it was removed from the market. Medeva Pharmaceuticals sells the name brand of phentermine called Ionamin and Gate Pharmaceuticals sells it as Adipex-P. Phentermine is also currently sold as a generic. Since the drug was approved in 1959 there have been almost no clinical studies performed. The most recent study was in 1990 which combined phentermine with fenfluramine or dexfenfluramine and became known as Fen-Phen.^{[citation needed]}

In 1997 after 24 cases of heart valve disease in Fen-Phen users, fenfluramine and dexfenfluramine were voluntarily taken off the market at the request of the FDA. Studies later proved that nearly 30% of people taking fenfluramine or dexfenfluramine had abnormal valve findings. The FDA did not ask manufacturers to remove phentermine from the market.

Phentermine is still available by itself in most countries, including the U.S. However, because it is similar to amphetamines, it is classified as a controlled substance in many countries (including Australia). Internationally, phentermine is a schedule IV drug under the Convention on Psychotropic Substances.^[1] In the United States, it is classified as a Schedule IV controlled substance under the Controlled Substances Act.

Phentermine is being studied with other medication for obesity. The experimental appetite suppressant drug Qnexa is a mixture of Phentermine and Topiramate. The FDA’s Endocrinologic and Metabolic Drugs Advisory Committee reviewed Qnexa on July 15, 2010. The committee voted narrowly against recommending approval.^[2]