no, the side effects are not related to how much weight you have to lose
it's related to your own reactions to the medication - as it is with any medication. for me, i would be stupid to take it, as even the one time i had red bull, i had a seriously scary reaction to it. phenergan (which is supposed to calm you down really) sent my heart rate out of control to the point that it was bordering on hospital and possible defib paddles to try and stop what it was doing. yes, i'm a big person but i have a very trim friend who has a similar reaction to phenergan. some children are sedated by phenergan, others go hyper

i wouldn't ever think that size would have any bearing on the number or severity of side effects from a medication