I gained in total 32 kg in three years from various meds. However I am a weightlifter, and love working out, so I utilised most (not all) of this gain to build more muscles. Perhaps it wasn't SUCH a smart decision as I now need not only new trousers but also t-shirts, shirts and jackets...

I spoke to my trainer at the gym and he said his girlfriend, a psychiatrist, was shocked to find out I am not ball-shaped on medication I am prescribed.
Whether you can entirely prevent it... I don't know. I'm sure you can make sure you don't balloon, but as CrackedGirl says a double combo of slowing your metabolism and giving you cravings (ah mirtazepine, the lovely pill that makes you crave sugar in the evenings...) will make it pretty difficult. But remember that as with all side effects weight gain is also something that some people get and some don't. You might be lucky -- hopefully that 8 lbs is all.

Big hugs and best wishes!
Currently working on my upcoming signature.