Effexor amongst other things is a dopamine reuptake inhibitor...

This means that it prevents your body from reabsorbing extra dopamine... (it tries to keep it in your brain longer to keep you happy)

Pot just so happens to boost your dopamine levels when you smoke it..

Because your body won't be able to reabsorb or balance out all that extra dopamine you made from getting high you could go into a seizure...

So I'd be damn sure that shit is outa your system before you go hitting a bong toke ;)