I've met players that smoke weed moderately to heavily and their success seems to follow no pattern.A bunch of successful and unsuccessful people do it, so I personally don't think it has a big positive or negative impact.  Using it while playing, before playing or after playing also seems to vary from player to player, so no patterns there that I've seen.I would focus on other health issues (staying in shape, eating decent food, getting good sleep) as I've seen a lot more impact in those areas.  While many great players stay up to all hours of the night and sleep into the evening at times, everybody that has gotten on a regular sleep schedule seems to mention how great it is and how much more energy they have to do things.While a lot of people that quit smoking weed do say the same thing, it seems to be more of them convincing themselves that it was the best decision (and often is) rather than any major difference.  One constant thing people that quit smoking report is that their short term memory improves quite a bit.  Feel free to do real research on your own, but I hear all the time from people that I respect that many studies on the subject are terribly biased either for or against the use of weed.I would use a rule of thumb with anything such as alcohol or weed or lack of sleep: If you do too much of it then you're much more likely to not be focused when playing and your play will likely suffer.So, if you like to smoke with your friend occassionally, I would doubt you will feel any major negative impact to your poker game.  If you're smoking up 24/7, then you might want to think about changing your lifestyle a bit. 

Great response Ryan!