Why am I tired on Phentermine?

Fatigue or tiredness can come as a result of sleepless nights or following a period of over-stimulation, however, adverse effects of phentermine also include drowsiness, fatigue, and reduced energy levels. If you’re feeling fatigued or lacking in energy while taking phentermine and the problem persists, you should consult your doctor as it may be that you have an undiagnosed medical condition.
For more information see: https://www.drugs.com/sfx/phentermine-side-effects.html