The positive side of diet pills to most people is that they achieve their objective of loosing weight. Adding "quickly" is the main key of the story lock, I must say.

 However, medical reviews are against the use of diet pills. It has recently been found that diet pills, even after many years of consumption, could cause a thickening of the heart valves in some cases. In most cases the thickening is mild and does not cause any health problems. In a few cases, this valve thickening can prevent the heart valves from closing properly (known as mitral or aortic incompetence). 

 Diet pills speed up the central nervous system (brain and spinal cord). Some weight-loss products contain stimulants or diuretics that can cause high blood pressure, irregular heart rate, sleep problems, nervousness, seizures, and even death. Excessive weight loss can be dangerous.