Hi, Klischia.
From what I know, most diet pills work on one or more of the
following modus operandi :
- they act as detoxification agents, 'flushing' out 'impurites'
(and also fluids) out of your body and since water weight is dense,
you'll end up appearing to 'lose a lot of weight'; or
- they interfere with the body's metabolism, containing chemicals
which increase your metabolic rate so that fat is burned at a
faster rate; or
- they work like Xenical, the famous drug that basically ensures
that any lipids are coated and then leaves the body without giving
it a chance to be digested and eventually assimilated as body fat.
The notorious side effect of this is - fecal incontinence, which
can be an extremely socially-crippling ailment and defeats the
purpose of helping you slim down cause who wants to be near a slim,
but fecal incontinent person, y'know? 
So, as you can see, all diet pills interfere with your natural body
processes to a certain extent. Different bodies will react
different to the same pills and produce different results. However,
you should pay due consideration to the side effects that these
pills may cause.
It isn't uncommon for the following side effects to occur :
dehydration, heart palpitations, nausea, giddiness, restlessness,
etc.
Basically, whenever you pop a pill, you're introducing foreign
substances into your body and depending on how efficient your liver
is, it may or may not be able to metabolise these agents well. Some
of these agents end up causing damage to the liver or
kidneys.
The big question is, is it worth it? Would you want to be plump and healthy with rosy cheeks
or slim but sickly and like the famous Lin Dai Yu of 'Dream of the
Red Chambers' (Hong2 Long2 Mong4)?
Doctors always recommend diet and exercise. A lifestyle change is
usually what's needed. Don't opt for the easy way out because you
just never know what the consequences might be.
From my personal experience, whenever we get a young patient with
abnormal liver function test results, I always hear the doctors
asking amongst themselves, "Is this patient on diet pills?"
Now, isn't that telling?
