Yes, phentermine can produce a false-positive for amphetamines on a drug screen. This is because phentermine has a chemical structure similar to that of amphetamines.
Phentermine is usually detectable in urine for about 4-7 days after the last dose, but may be measurable in hair samples for much longer. It will also stay in the urine for longer if you were taking a very high dose or have been taking phentermine for a long time.
To read more about phentermine and drug tests, please click here.