I don't know about toxicity and stuff, but at the end of the day, you are putting an alien fume in your body with sole intent to temporarily modify the state of your psyche. For people who may already have some sort of condition, however unnoticeable in day to day life, or maybe they don't even know they have it or it could be a very diluted condition, whatever, cannabis is going to effect your psyche slighty (or majorly if the condition is serious) differently from someone without a condition. Now personally I think to some extent most people have at least several conditions, even if it's just something like being a chocaholic or a mildly paranoid individual, so certain people will not take cannabis very well, especially when in large amounts.
A large amount of cannabis can do some pretty sketchy stuff and I've had paranoia and anxiety attacks after a lot of weed (only when high personally) so there's a very good chance that someone more psychologically messed up than me, someone who doesn't take weed as well as me or whatever could be quite dangerously effected.