An EKG is a screening test to make sure your heart is in a normal rhythm and no irregularities are found. The extra stimulation from the effects of phentermine makes the cells in the heart more susceptible to changes in electrical impulses. It is important to make sure there is no underlying cardiovascular problem that can be exacerbated. If your heart rate or rhythm is abnormal, you will be referred to cardiology for clearance before starting phentermine.