Scientists confirm that marijuana, whether medical or recreational, is safer than both alcohol and tobacco. One of the considerations that went into this statement has to do with the lethal dose determinations.
When it comes to determining the danger of various drugs, there is a need to compare the lethal dose to the standard dose. How much do you need to get high, versus how much will kill you. One of the dangers of addiction with a chemical tolerance component, has to do with the fact that a higher tolerance means a higher consumption. In the case of drugs with a low ration of needed to get high to lethal dose, increased consumption can mean higher fatalities due to what people call overdose.