CB2 activity, especially of "lesser" cannabinoids, accounts for a lot of weed's pharmacological profile. There are of course the subjective mental effects, fatigue and sedation, but physiologically, it appears cardiovascular pressure is moderately reduced (vasodialation), whereas cerebral blood flow is noticably increased, heartrate and plasma content increases, and mild muscle relaxation occurs. The sum total of which is not nearly equivalent to the action strong CNS depressing agents such as GHB or heroin have to slow down respiration and heartrate, to the point at which at a high dose the heart simply stops, and they may have syngergistic action on eachother, so safely dosing a combination accordingly is difficult and still dangerous. However, people can pass out on weed, though this is unrelated to its depressant activity, know that during sleep your breathing and heartrate also slows down and this can be dangerous.