This is a very important question. Yes, marijuana is bad — very bad. There are many reasons.
1) It causes mental health problems. Most commonly, users experience what they call “paranoia” — basically irrational anxiety. Some users have panic attacks, which can last even after you stop smoking pot (and panic attacks are really no fun — you feel like you’re going to die). The thing is, a large proportion of the population has some level of susceptibility to anxiety problems, but many people never develop them. Weed, for some reason, seems to bring it to the surface. Since tranquility of mind is such an important value, both in secular and religious terms, no one should ever use marijuana (unless as a last resort and prescribed by a doctor for a pain condition, but I wouldn’t use it even then.)
2) This may sound surprising, but 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. Schitzophrenia is a relatively common but horribly devastating disease. It really ruins people’s lives. Another reason to stay away from marijuana at all costs.
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. I know many pot smokers who stopped, and felt themselves gradually coming out of a fog, their mind becoming clearer and clearer.
5) Pot users tend to hang out with people who do other drugs, which are just as or even more dangerous. I know many people who have messed up their lives and given themselves lifelong anxiety and even vision problems through using psychedelic drugs. Nowadays tons of people take opiate pills and end up on heroin.
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.
Why did Hashem make this plant? Well He didn’t make it with lots of THC in it. The original plant only has a bit. It’s very useful for making fabrics. People bred it to increase the THC. Why did Hashem create THC? Perhaps because it would have some medical use someday. Perhaps also as a nisayon?
Can you overdose? Not technically (as with heroin), but in practice, it happens all the time that people smoke so much they get really out of it and paranoid — a very unpleasant experience that most pot smokers will admit to having experienced. (That said, smoking just a little can trigger anxiety attacks too.) If you ingest it instead of smoking, you can also get way too much THC and feel like you’re going out of your mind. It’s harder to regulate the dose that way, but people eat pot brownies anyway.
Synthetic cannabis is even worse — don’t even think about it. People are jumping out of buildings and killing people and such because it makes them go nuts. If you hear about some drug, synthetic or not, that’s legal, don’t go order it just to see what it’s like — I know people who have suffered greatly from this mistake.
All the examples I’m using are from secular people I’ve known, not frum yidden. In Israel I’ve heard there’s some chassidic BTs who smoke pot, but the rabbis denounce them and I gather they’re an embarrassment.
Some people want to use pot because they crave some kind of altered consciousness or experience. Some even want to experience spirituality through it. Don’t count on it. Jews have relied on prayer, meditation, fasting, and a little alcohol to bring on altered states. That’s all we need. If you’re interested in expanding your consciousness, rely on our historical methods, not drugs that can mess up your mind.