I was hospitalized after taking LSD while also on Lithium. LSD is incredibly safe, unless paired with Lithium, so it was a bit of an unexpected outcome. I ended up in a state where I appeared unconscious, but was very much still, and experienced excruciating seizures.
Otherwise, what I saw with people going the wrong way with other substances involved quite a bit of pain, vomiting and enough distress to leave a definite negative impression. Even with heroin which many people think might be an easy out, it is not as peaceful as I understand a morphine od is