Exacerbation (and possibly development) of mental illness like psychosis, bipolar disorder, or schizophrenia is entirely possible.

Along with two other blokes, I got interested in psychedelics in high school. Took one medium high dose and wasn't right for a few months. Never in my life did i ever experienced paranoia, delusions, or hallucinations that are genuinely hard to separate from reality, but I did after that.

Intense psychedelic experiences can fracture what you once knew as "reality" allows all sorts of ideas to float into your mind, with equal possibility. This might be helpful and give you more flexible thinking (helpful for depression) but it also leaves you incredible vulnerable to all sorts of garbage ideas that you never would have considered otherwise. ie conspiracy theories or straight up delusions about the supernatural. Remember: It's not paranoia if you genuinely believe they really are out to get you!

Fighting these garbage ideas is a lot of work once they take hold, but you'll only know too late if you were vulnerable, and worse, if you can successfully align your understanding of reality with most other people.

I got extremely lucky that I stabilized. I'm convinced part of this was only doing it one time. My two co-experimenters took many trips with various doses are still in and out of mental hospitals years later. Psychedelics are incredibly potent and nobody really understands them very well. A lot of what is written on the internet ignores, downplays, or denies the very serious risks to your philosophy of mind and mental function. Its like playing with fire when you don't have heat sensation in your hands.

Several other comments on this page echo these warnings. One even claims there is an 18% chance you could go from depressed to schizophrenic. I have no idea where that figure came from, but the risk is certainly not 0%