By any reasonable standard, marijuana is a mild drug and as for overdosing, there is no scientifically valid evidence of anyone dying of an overdose of marijuana smoke. Of course, it is possible to commit suicide by consuming large amounts of marijuana. But it is possible to die by eating too much salt. Salt is not illegal. Aspirin kills by overdose and that's legal.
Many people die by drinking too much alcohol, an addictive drug.
It too is legal. One argument made against the legalization of marijuana is that it damages not only the user but innocent bystanders. This argument, like the one about protecting the user, has two parts. The first deals with physical injury and the second with spiritual health. The main physical threat to society is that users under the influence of a drug with crash a car or airplane, or lose control in some way and do harm.