What drugs/ illness can make someone have false beliefs?

Mental illnesses..? Probably either schizophrenia, bipolar 1 with psychotic features, or schizoaffective. Drugs..? A hallucinogen could do it. Just about any, if you're vulnerable to psychosis. (Although it doesn't TYPICALLY happen, if you're not. And I've personally had EXTREMELY good, and positively life-changing, experiences with it!) In fact, MAYBE even high doses of weed, if you're someone who easily becomes psychotic, could potentially push you over the edge. And also, definitely stimulants, like cocaine and amphetamines! Stimulant psychosis is TOTALLY a thing! Especially if you abuse them, and use lots and LOTS of them, in high doses.

Anyway... In my experience, stimulants are actually substantially MORE likely to make people psychotic than hallucinogens! A friend of mine became psychotic from them, and now has a diagnosis of schizophrenia, whereas she was previously NOT mentally ill. (You never know, though... It just might have hit, anyway. She was at an age where many people develop psychotic mental illness, IF they are going to develop it.) While the love of my life..? Has bipolar 1, with strong psychotic symptoms, and has still had many really positive psychedelic experiences, that were helpful to him! But also..? Had to quit his stimulants, that he was taking because he ALSO has pretty severe ADHD, because they made him manic and psychotic, to the point that it landed him in the psych ward.

So yeah... There you go. Hope that was helpful.

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Magic mushrooms the cheapest way to get high for a few hours.

There are poison (drugs), grief, cancer, Alzheimer (old age), trauma, infections, parasites and birth defects that can alter our perception of reality.

The brain is by far the most interesting thing in the world. We can experience things for some reason and that's all we'll ever know for sure. (Unless you experiment on your own brain.)