An overdose on hallucinogens can cause various issues like respiratory depression (slowed breathing or trouble breathing), coma, convulsions or seizures, and respiratory arrest (which is lethal). Other long term affects can be visual disturbances and paranoia.

Hallucinogens alter the brains sensory input and alters its ability to comprehend what you may be seeing, hearing, or feeling. Some hallucinogens also target the brain's serotonin, which affects your mood, hunger, sleep, and sexual behavior.