Maybe that's not a horrible argument in general though, everyone and every body is different. Some things are more therapeutic and medicinal to some people because of their unique physical and mental composition, and other things are more toxic.

I found out I'm a slow metabolizer of caffeine, a cup of coffee will last like 16-18 hours for me, even though the half-life is supposed to be 3-7 hours for adults, which makes coffee a more dangerous drug for me because it'll disrupt my sleep more than the average joe. I still like coffee but I have to be more careful than most people.

That being said I'm pretty sure cannabis is horrible for the developing brain. I remember one anecdote of someone who told me they would get really high when they were 14, and would get so high that they would literally black out, like have no idea what just happened. Probably best to be avoided until adulthood. Nobody when I was in college or older ever experienced anything like that, just the usual report of infrequent users forgetting what they were just talking about or what someone else just said to them, the whole short term memory loss thing.