So you are talking about illicit drugs? Likely, none at at all. At least in the US, drug use is not considered sufficient to diagnose a mental illness. I'm honestly kind of confused by your question. What is a mental illness incident? I've never heard this term.

Have you actually googled this at all? This reads like a question made in total ignorance of legal recreational substances, as well as their effects.

I'm trying not to be insulting, but as someone who's social circle nearly universally consumes recreational drugs (not counting tobacco or alcohol), this comment is cringe-inducing how out of touch with reality it is.

To answer your question, 0% can be attributed to it. Alcohol has been legal forever, but medicine still recognizes what alcohol-related addiction, psychosis, and liver cirrhosis are. Medical diagnoses don't really care if the drug involved in a drug related symptom is legal to purchase or not. There are people who don't consume drugs of any kind and have many mental illnesses, and there are people who consume many drugs and don't have any mentally illnesses at all.

It is well documented that marijuana can cause psychosis in some people that are predisposed for it. I know quite a few people, my self included, that started using cannabis way more often after it was legalized.

I think it's reasonable too assume that legalization of a formerly forbidden substance will increase overall use of that substance. I also think that it is reasonable to assume that increased use of a substance will increase the amount of people suffering the potential side effects.

That said, I think social media, the economy, the pandemic, pollution, and the war on terror drugs have done way more harm than legalized weed.

Psychotropic pharmaceuticals can have strong interactions with legal and illegal drugs alike. If you're taking most types of these, you won't want to smoke pot or drink alcohol. Also, since you can't depend on the ingredients and dosages of illegal or unregulated drugs, there is really no telling what drugs you could or couldn't take in concert with your psychotropics.

And then there are questions of whether you'll tell your psychiatrist. I've seen a quite accepting, non-judgemental attitude about recreational drugs, but can you count on this to be consistent? Do you want this on record in your medical chart? Do you want to try and consistently lie about activities quite germane to your mental health?