Diet pills are full of caffeine (or other things) that raise your metabolism. Taking a pill may hasten weight loss, but your metabolism will drop when the drugs fade and soon your body will build a tolerance.
This means, for someone who wants to continue using the pills, you'll have to take more and more pills to get and keep your metabolism up.
This also means that if you stop taking the pills, your metabolism will slow drastically, making you feel like shit and probably gain the weight back.
So, unless you're committed to shovelling money over the counter and shovelling pills into your mouth for years to come, pills are only a futile method to lose weight temporarily.
Many diet pills are not FDA approved, and as we learned with ephedrine, that can turn out to be very dangerous.
Laxatives do not aid weight loss.
If you're looking to take diet pills as a source of energy, you're probably going to be paying for caffeine. caffeine will raise your metabolism and blood pressure, giving you energy, possibly making you shakey and irritable, and leaving you very tired once it wears off.
If you're stuck on the idea of taking some kind of pill or getting energy from a source other than food, I recommend herba mate tea, a natural energizer, or green tea, which contains caffeine but at a much lower level than coffee.
If you're not undernourished, exercise can help up your energy level, but if you're undernourished and underweight this will ultimately just make you more exhausted.
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