Well, there IS a theoretical LD50; an amount which will be lethal to 50% of individuals to whom such a dose is administered. For the primary active components of marijuana - the cannabinoids - this level is impossibly high. You'd have to smoke more than your body weight in high-quality stuff to get anything approaching this dosage. You're far more likely to die of suffocation due to acute pulmonary failure while attempting to smoke that much pot. That's never happened, to my knowledge. Which is not to say that no on has ever died from smoking weed. In fact, the primary cause of marijuana-related deaths is cardiac failure, particularly in individuals with a prior history of heart or blood pressure problems. Marijuana causes blood pressure and pulse rate spikes, which can be dangerous for those with susceptible conditions.