HomePhentermine FAQsIs phentermine safe?Is phentermine safe?Phentermine is an FDA-approved medication, and is generally safe when taken as prescribed. Phentermine, like all other medications, can cause side effects. However, when someone is prescribed a weight loss medication such as phentermine, it is because your doctor feels that the benefits of losing weight to reach a healthy BMI outweigh the possible side effects.Phentermine is different than Fen-Phen. Fen-Phen (aka Phen-Fen) was a combination of phentermine and fenfluramine that, after being widely prescribed in the 1990s, was found to cause serious cardiopulmonary side effects. The combination is no longer prescribed and fenfluramine has been withdrawn from the market.→ Considering phentermine? Read the ultimate guide to learn all you need to know this medication!