Phentermine is an FDA-approved medication, and is perfectly safe as long as it is taken as prescribed. It is true that 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 not to be confused with Fen-Phen, a combination of phentermine and fenfluramine which caused serious side effects after being widely prescribed in the 1990s, and which has since been withdrawn. You can find out more about Fen-Phen here.