Honestly, bloodshed is spouting off side effects that are possible side effects. Not everyone who takes an SSRI will have trouble orgasming or other sides and so on and so forth.. you can google just about any symptom in relation to any drug and find a few hits. Google hallucinogens and HPPD or talk to someone like Magicduck. Obviously if you're on an SSRI there is a reason and you and/or your therapist deemed it beneficial for you.
Personally I was on prozak for a few months before I realized it wasn't for me. I had an amazing amount of insights and in some ways I really liked the person I became, I felt more like me if that even makes any sense. It was a very eye opening experience. However, I soon found myself unable to hold opinions or feelings about anything, even stuff I enjoyed, everything was meh, but I was happy at the same time. I just started not to care about anything and it was bad for me. Also the lack of being able to orgasm affected me, and the prozak infact made me even hornier than ever.. so this was a big problem. I was seeing girls left and right but I couldn't nut.. no thanks. I also ended up breaking out with pretty bad bacne and chestne, something I had never had in my life before and which I 100% associate with the prozac.
I don't know the exact science behind it, but basically the medication you are on targets receptors in the brain that the drug you want to take also targets, because of this the drugs will not effect you in the same way it would a normal person, and could even be bad for you. In a nutshell your SSRI fucks with the target that the drugs mechanism is tryin to do work on.
As said your weight has nothing to do with how the drug will affect you really, and in all seriousness you should start out with 500ug to gauge your reaction to the drug..
However as has been said, you really should consider getting off the SSRI if you really want to use substances like these.. Don't listen to all the horror stories about antidepressants online, make the decision for yourself. Are they working for you?
Personally.. I actually really liked the person I became on them.. its just the negatives became worse than the benefits. Life is always a balancing act my friend