How harmful can cannabis be to a person with existing mental health problems?

There's a correlation between marijuana use and psychotic breaks and schizophrenia in some people. If you have mental health issues, it's best to avoid it.

Cannabis isn't for everyone. Some people experience anxiety and paranoia while they're high. I've seen this happen to people who don't have mental health problems so if you do and you're considering trying it, take it slow. Very slow. Don't eat edibles because you don't know how strong the cannabis is. Second, if you do start to feel paranoid realize it is the weed and it will pass.

I've not had these problems and I know people who DO have mental issues and use it without problems. It's your call, just be cautious.

some of it is very strong and can distort the mind greatly