There's an LD50 for THC tested in rats, and it's actually less than pure alcohol (meaning that pure THC is more poisonous than pure alcohol). Thing is, there's much less THC in marijuana than there is alcohol in an alcoholic beverage. You could easily kill yourself with pure THC, but as for the actual plant, a person would be too dozy to smoke more long before it got toxic, and as for eating it, the human stomach simply won't hold enough to kill someone. It's the same reason you can kill yourself by swallowing a bottle of caffeine pills, but not by drinking a gallon of coffee--there's not enough room in your stomach for a toxic dose of coffee. But of course that's just acute toxicity. Long-term, smoking anything will damage your lungs. Plus there's the possibility that it changes brain chemistry after prolonged heavy use.
But this has been gone over a dozen times. Can we agree that acute toxicity really isn't the issue here? Long-term exposure, side effects, interactions with other drugs, and stoned drivers with slowed reaction times are much more of an issue than that. And it does suppress nausea, which is a good thing for chemotherapy patients and people with AIDS, but not too good for someone who needs to throw up to get rid of, say, a bellyful of too much alcohol.
It's not safe. It's not unacceptably dangerous either. We have to weigh not just the risks of legalizing it, but the risks of keeping it illegal. Whichever one actually happens, I have no desire to try it; it just doesn't appeal to me. Even medicinal use may come with unacceptable side effects for many people. When we take psych meds and they make us feel sleepy and loopy, we don't usually like that because we're trying to get on with our lives. Same goes for medical marijuana--those side effects might be desirable for recreational users, but they're not going to be very useful for everyday life. Maybe a chemical can be found that is similar to the naturally-occurring stuff, which allows for the positive effects without the undesirable change in cognition.