What are the long term side effects of hallucinogenic drugs?

Depends on the drug: Some hallucinogenic drugs such as lsd can produce flashback symptoms (hallucinogen persisting perception disorder) for months or years after ingestion. This may or may not be interspersed with normal behavior. Marijuana is also regarded as a hallucinogenic drug, and may cause persistent psychosis as well as learning, memory, and behavior problems long after the use has stopped.