I just realized my previous comment probably wasn’t approved because it was too long — sorry about that. Here’s the short and sweet version:
Yes, cannabis is bad — very bad.
1) It causes mental health problems, including paranoia, anxiety attacks, and more general anxiety problems. This is extremely common, and experienced by most users at one time or another. Ask any user about paranoia, and they will know what you are talking about.
2) There is very strong statistical evidence that using cannabis increases the risk of schitzophrenia and psychosis. These disorders are caused by an interplay between genetics and environment. For example, when one identical twin has schitzophrenia, the other twin only has about a 50% chance of coming down with it, even though they have all the same genes. So environment matters. And smoking marijuana is one thing that triggers it.
3) Pot can also cause burnout. I have personally known people who after using a lot of weed kind of lose their minds. They don’t make much sense, and don’t have much intelligence anymore. I taught a class last year (most of the class were secular non-Jews), and I asked them if anyone knew someone who was burnt out. Many said they did. It’s very sad.
4) It clouds your mind and lowers your memory and intelligence.
There is a dispute about whether it was part of the ketores. R’ Aryeh Kaplan says yes, but others argue no. I’ve never seen any evidence that any Jews smoked marijuana historically. It has certainly never been used by any great Torah scholars or tzaddikim.