I have an old prescription of phentermine, would it be harmful to use it?

I would do a web search re: the efficacy timeline for the drug. Many drugs last for decades with the only issue being loss of potency. Some drugs however, could deteriorate into toxic ingredients. The military did a study on some antibiotics in their supply chain and found them still effective and safe 20 years later. Do some research before you decide.

Do you still have it?

The best bet is to call your pharmacy and inquire about expiration. Call a couple to compare answers.