Phentermine is similar to amphetamine, as we’ve established. Thus, a drug test could display a false positive. But, there’s no need to worry because you won’t actually test positive for amphetamine. If you don’t want the risk at all, please note that a urine analysis after four to seven days of the last consumption should have clean results. Of course, this depends on the frequency and amount of dosage, as well as the body’s metabolism.