I was once given these tablets by someone at work who did not want to take them anymore because they did not agree with her.
I took them for 3 days, side effects were palpitations, so badly that I found it scary. When I looked in the mirror I noticed that my eyes were wider and I did not sleep a wink (this is from coffee girl, who drinks very strong coffee before bed and sleeps like a baby). The reason I stopped were the palpitations.
I lost about 5 lbs in 3 days. (in total I had 2 stone to loose to get to healthy weight)
I would not recommend them, but if you are going to do it anyway please make sure they are prescribed by a reliable doctor who decides to give them to you because they are suitable for you rather than one of those slimming clinics who prescribe everything to anyone because it is easy money.