There are harmful effects of some diet pills. It becomes a research assigment in finding the best diet pill that will work the best and be the safest for you. Some diet pills, especially the stimulant-based diet pills, can be habit forming and as such can be abused. Abuse of these certain drugs, may lead to a condition called dependence. You should ALWAYS check with your doctor before taking any prescription diet pill to find out if they are habit forming or not. Your doctore will be able to steer you in the right direction.