My classmates, which are started to smoke marijuana in school, now are dead (I am 40 years old). They die not because of tar or something like that, but because they switched to heavy drugs.
It is fact from my life. 4 my classmates started to smoke in school, when they were 14 years old. Then somebody told them how to make cheap drug from papaver, which is called «shirka» there. Now they are dead.
An argument I would attempt to counter - the market is the gateway. The dealer you buy your illegal cannabis from often also sells 'harder' drugs, such as cocaine and heroin.