I would avoid Clenbutrex. Of all the fat burners I have taken, Clenbutrex was the only one that fucked me up. It actually made my thyroid gland swell which caused great difficulty in swallowing. I have taken Hydroxycut, Ripped Fuel, Xenedrine, Dymedrine Extreme, EPH, prescription ephedrine, etc. and no side effects from any. I dont know what is in Clenbutrex that didnt agree with my body, but I'll never take it again.
Oh, personally the best fat burning combo I ever used was prescription ephedrine, vivarin and aspirin. I went from 13% to 9% bf.
Later...