The first example that I found quite interesting is hallucinogen persisting perception disorder (HPPD), which sounds like it could be extremely inconvenient. After as little as a single dose of LSD, psilocybin or related hallucinogenic drugs, symptoms of visual aberrations persist. Hallucinating for the rest of your life takes the fun out hallucinations. Sufferers of HPPD also can distinguish what is a hallucination and what is real ie. they have pseudohallucinations, which sounds even less fun.
The second example involves a illicit synthetic opiate drug called (1-methyl-4-phenylpiperidin-4-yl) propanoate (MPPP).
From an article in TIME magazine:
When George Carillo arrived at the Santa Clara Valley Medical Center in San Jose one steamy July day in 1982, he seemed more a mannequin than a man. The 42-year-old heroin addict was bent over and twisted, drooling and unable to speak; almost every muscle was immobilized. No one knew what to make of his condition, so a call went out for Dr. J. William Langston, the hospital's chief neurologist. Langston took one look and was amazed. Carillo's symptoms suggested that he had been suffering for at least a decade from Parkinson's disease, a nervous system disorder that causes tremors and a gradual loss of mobility. But that hardly seemed plausible: Parkinson's rarely strikes anyone under the age of 50.
Using stiffened fingers to scrawl answers to doctors' questions, Carillo managed to provide a few clues. The symptoms had come on suddenly after he and his girlfriend, Juanita Lopez, 3l, had tried a new synthetic heroin. Though the drug had caused an odd burning sensation when injected and hallucinations, they continued to use it for three days; two days later both had frozen into living statues.During the manufacture of MPPP the related MPTP can be accidentally produced. MPTP in the body is converted to a neurotoxin which is selectively causes neuronal death in dopaminergic cells. This selective toxicity causes the hallmark Parkinson's symptoms. This drug has been now used extensively to study Parkinson's and create animals models of this terrible disease. It also hints that an environmental toxin may contribute strongly to early onset Parkinson's. The possible causes of Parkinson's is extremely interesting and warrants a separate post.
There is a book available, "Case of the Frozen Addicts" and a NOVA documentary of the same name about MPTP victims. There are at least 3o0 people in California who have used this drug. I can't find a download or streaming of the NOVA episode unfortunately.
Of course, medically prescribed drugs can have some extremely bad permanent effects, but hopefully in that case there is a professional who knows the drugs and what to look out for. Also recreational drugs are done on a self dosing schedule and, especially in the case of addition and/or dependence, one might loose perspective on symptoms of damage and subsequently an appropriate dose.