Because marijuana was mentioned in a casual use context in another thread, this seemed like a good time to share my, albeit modern, view. I was recently awarded disability for nerve damage and IBS among other things. The pain has gotten so bad that nothing short of a Demerol shot in the emergency room provides any significant relief. Over a year ago, I caved and tried marijuana. For the first time in a decade, I got my mind back. It wasn't riddled with streaks of white pain. In my case there would be no chance of meditation, focus, or reflection without hindering my senses with something.
For me, the 5th precept is relative. If use of any substance hindered my growth, I would enact the precept automatically. Another thing to consider is the idea that current Sangha are likely very experienced. They are on their way out of this plane. They would also not have the baggage that the rest of us lay people do. I hope that whatever karma I'm paying back this cycle allows me to be in the Sangha next round. This view also gives me great peace. I don't feel my ego activate which is always a good sign.
But, I'm new this go around, so I yield to the more experienced.