Let me just respond to a few points in this thread. About 11 years ago when I was in law school I worked on a criminal case involving hemp vs scheduled marijuana (different THC levels and cops do not test that) so my detailed scientific research may be outdated:
1. The "monkey" study involved 5 monkeys who were essentially gas masked with hoses that only spit out marijuana smoke. The monkeys all died after 5 minutes from massive brain damage which was the basis for the claim that weed kills brain cells. What this study neglects to highlight is that the monkeys were breathing in nothing but smoke and died from oxygen deprivation which is what killed their brain cells. As for motivation, if pot makes you unmotiavetd then you are a lazy Type B personality to start off with. I can smoke weed and am up for anything. I have never smoked before a workout however.
2. Positive effects from marijuana include blood vessel dialation which increases blood flow to your muscles. This helps with healing and thus building. In addition, it also relaxes your muscles which leads to the same benefit. As for vitamins, always take a multivitamin and Omega 3-6-9 supps. Remember, marijuana was used for thousands of years as a medicinal drug (moderation) and was outlawed in the 1920s with the political influence of the Dupont family who feared competition from hemp fibres with their synthetic fabrics we all know and love.
Frankly, this is a non-issue unless it makes you eat like a pig. You are wasting time thinking about this when you could be training. If you don't stop, I will find you and smoke you up!