It isn't if you use a vaporizer and are mentally healthy, but even if it were indeed unhealthy (at least more unhealthy than junk food or whatever comparison you'd like to make), it's the decision of the individual what he puts into his body. So the discussion if it's healthy or not isn't relevant on this topic.
It's not physically addictive and is healthier than legal drugs like beer, cigarettes and coffee.
I think the jury is still out. You might be interested in this article in the Journal of Alzheimer's Disease (I hope the link works, I'm not too sure if I entered it correctly on this forum).
https://www.j-alz.com/content/new-study-shows-marijuana-users-have-low-blood-flow-brain
Even if it was, I think it's people's right to do with their bodies what they wish. Let the free market do as it will.
It isn't if you use a vaporizer and are mentally healthy, but even if it were indeed unhealthy (at least more unhealthy than junk food or whatever comparison you'd like to make), it's the decision of the individual what he puts into his body. So the discussion if it's healthy or not isn't relevant on this topic.