having a low IQ isnt a mental health problem.....
That's true. However your suggestion is that cannabis use damages the brain.
There are plenty of studies which suggest cannabis use damages the brain, plenty of studies which suggest cannabs use doesn't damage the brain. Basically these studies don't really do anything except suggest more research is required.
The recent study of cannabis use and IQ was one of those studies which is authoritative which is why I mentioned it. They studied a lot of people and followed them up for some time. The conclusions were that young teens can damage their brain smoking weed (assuming lower IQ = brain damage) but by the time you reach 18 then cannabis is pretty safe stuff.
If you take this message back to the earlier studies which aren't so authoritative then it helps explain the results. The brain is developing until late teens and cannabis use does seem to affect this development in a negative way. Once you are a tad older, 18 was the year given, you aren't really at risk.
However it's certainly one of the safest recreational drugs out there and a lot safer than tobacco (or booze).