The active ingredients in most of these diet pills are ephedrine and large amounts of caffeine. Ephedrine is now illegal in Canada. A few years ago (before it was made illegal), I got stuck in the cycle of taking diet pills.
What CadMan said is bang on. There are far too many negative side effects, and dangers associated with taking diet pills.
My own experiences with them were not good. Yes - at that time period, I was the slimmest I have ever been... but at a cost. I was irratable much of the time (after the intial euphoria), my heart would race, I would get cold sweats, headaches, and insomnia. lol I should mention that I was fired from a supervisory position at that time in my life for an outburst with a co worker I was supervising!
I was taking 1-2 a day before my workouts... and that's how it started. Then I started taking one before work. I was working 12 hrs a day everyday (2 jobs), and felt like I could not go a day without that extra energy.
They are addictive, and there are actually physical withdrawal symptoms when you stop taking them. I took them for a year and a half.
In addition to all the negative effects that other posters and I have mentioned, there are also 3 more to consider:
They wear out your adrenal glands.. and can seriously fuck with your metabolism long term (it will take a while to get back to normal when you discontinue taking them).
Diet pills can actually inhibit muscle growth.
Diet pills can cause permanent Tinnitus.
Long term effects for me?? - I have permanant Tinnitus, and an ear condition.
I have not taken any diet pills in years, and never would again.