Are diet pills effective weight loss tools?

Probably not. Over-the-counter diet pills decrease one's appetite a bit (they are related to stimulants). A person's desire to eat can be much stronger that a diet pill's power to decrease his appetite, so he will likely continue to eat too many calories. Also, diet pills don't increase one's desire to exercise. If something as simple as a diet pill actually worked, we would not have so much obesity in america.
Yes. Under medical supervision, a proper weight loss program which includes a healthy diet and exercise plan, some diet pills can be effective tools for weight loss.
Yes. Diet pills are more effective when combined with lifestyle modification including proper nutrition and exercise. In most instances, diet pills lose their effectiveness after a period of time.