My doctor has me on 37.5 phentermine 2x a day. she gave me didrex 50mg 3x a day to start on the day i run out of phentermine. any issues with that?

Not recommended : The cardiovascular risks outweigh any benefit of these medications.

Concern: Phentermine 37.5mg is the usual maximum daily dose. I am not clear why you need Didrex or why you will run out of phentermine. Combination of the 2 could lead to hypertensive crisis or tachy-arrythimia and will increase risk of complications. Avoid pregnancy while on phentermin

Okay to switch: Phentermine and Didrex are in the same class--both are basically analogs of amphetamines. Some people respond to one drug better than another. The main disadvantage of Didrex is its cost.

Weight loss: That much Phentermine can and will affect your sleep cycle. Also, please stop the Phentermine 2 weeks before any surgery to avoid cardiac arrythmias and possible death.