Will grapefruit juice effect the way phentermine works?

Grapefruit juice interferes with a lot of meds. phentermine is not one that I know of. To be safe you need to check the label on the bottle. It will state that you should avoid grapefruit juice on it. Also read the paperwork that came with your prescription carefully. It will state it in there, as well. To be safe call the pharmacist, as they will able to advise you further.

I would avoid drinking grapefruit juice while taking phentermine until you are sure so you do not place yourself in harm's way.