inhumandecency says:

If it helps, the research I'm aware of regarding "flashbacks" has been done under the rubric of "post-hallucinogenic perceptual disorder" or PHPD.

Anecdotally, I know quite a few people who have used quite a lot of drugs, and they don't have the kind of widespread problems with flashbacks this author suggests. I suspect that he's highballing the figure, of course, but there may also be qualities of the experience, the user, or the particular psychedelics used that contribute to determining risk of flashbacks. I understand that flashbacks also range from vivid but harmless memories to experiences closer to spontaneous drug trips, and useful research on the phenomenon would have to make that distinction as well. It's a shame this kind of important health-relevant question is usually buried underneath the political necessity of appearing "tough on drugs."

inhumandecency says:

There's some discussion of PHPD on the mailing list for MAPS, a great pro-psychedelic-research organization (which also helped with funding and governmental approval for Dr. Moreno's psilocybin / OCD research). Go to http://www.maps.org/forum/search.html and search for PHPD. The mailing list attracts a mixed bag of researchers, dilettantes, and eccentrics, but it often has pointers to published research.

Simon says:

PHPD is also known as HPPD and is (at least according to some) a different category of perceptual distortions than flashbacks. Whereas flashbacks refer to episodic disturbances in thought and/or vision, PHPD/HPPD usually refers to permanent long-term visual distortion following psychedelic drug use. I believe Dr. Paul Blenkiron is pulling his information from McGlothlin WM, Arnold D: LSD revisited-a ten-year follow-up of medical LSD use. Arch Gen Psychiat 24:35-49, 1971. I can't find this article online, but this article has a nice summary of the data:

A number of authors have claimed that frequent use of LSD is more predisposing to flashbacks than rare or occasional use of this drug (1, 6, 9, 10, 11, 15). Blumenfield (1) found that flashbacks were reported more frequently among heavier users. McGlothlin and Arnold (9) also presented data to support this point. Of their subjects who had used LSD fewer than ten times, 12 percent reported LSD-like recurrences; of those who had used it ten times or more 24 percent reported these recurrences. On the other hand, several different analyses of our own data did not support this finding. The figures we obtained were 28 percent for those who had used LSD one to ten times and 30 percent for those who had used it 11 times or more; the two figures did not differ significantly.

summer says:

HPPD/PHPD further clouds diagnostic lines in schizophrenia. People whose psychosis was initiated by drug use are 1. more likely to have visual hallucinations; 2. have shrinks raise their dosage when they claim risperdal is making the symptoms worse. Some research supports this:
"LSD-like panic from risperidone in post-LSD visual disorder"
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=8784656&query_hl=4&itool=pubmed_docsum

Regarding frequency of flashbacks, I'm going to guess and say that it only takes one bad trip for someone with a genetic predisposition to "schizophrenia" to develop a full-blown psychotic epsiode (and eventually schizophrenia), whereas someone without that predisposition is more likely to suffer residual visual disturbances after repeated drug use.

I'm about to read this review: "Hallucinogen persisting perception disorder: what do we know after 50 years?"
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12609692&dopt=Citation

summer says:

I have to share the following line from the review linked above (Hallucinogen persisting perception disorder: what do we know after 50 years? Halpern, 2002):

"some cases have been reported to improve with the use of sunglasses (Abraham, 1983)"

daksya says:

About flashbacks, I would be skeptical of percentage rates in the double digits. As per the US national survey (NSDUH), typically 10% of age 12+ are estimated to have tried LSD i.e. ~25 million. From an earlier administered survey when such questions were asked, typically 70% of users have used upto 10 times in their lifetime, and 30% 11 or more. So, if Blenkiron is correct, then roughly 4 million Americans have experienced flashbacks, and yet unlike other conditions with similar prevalences , there's only one support group that I know of for flashbacks/HPPD and one specialist in the entire US. This is not to deny that no such phenomena exists (although I or my friends have never experienced it) but either their severity is low or the prevalence is nowhere near as much as quoted.

It should also be noted that apart from the internationally (?in?)famous practice in the Netherlands of tolerating cannabis sale in coffeeshops, there are other shops (roughly 150) that sell psilocybin mushrooms and mescaline-containing cacti and have been doing so for a decade or a bit more now. In 2000, an agency that reports to the Dutch Ministry of Health, upon commission, reviewed the risks of 'paddos' (mushrooms) and declared them as posing a low risk to individual and public health, based on the literature and review of the magnitude and severity of cases of toxicity encountered in the Netherlands till then.

Malgwyn says:

What is commonly referred to as flashback" seem to be more like a panic attack. The imprint experience creates a chemical imbalance that is memorable and intense. Later chemical imbalances, of whatever cause, are interpreted as connected to the previous experience(s).

It may simply be a matter of sensitivity. A person with no past psychedic experience could experience a nearly identical chemical imbalance, and might barely notice it.

adhesivesynergy says:

I agree with Malgwyn, as someone who has used psychedelic substances in the past I've experienced something you could describe as 'flashbacks', often at random. I think neuroscience has a long way to go before it will be able to tell us how certain brain states lead to subjective experience (if it ever will, and without the hand waving that has filled the field since Skinner). At the risk of sounding unfashionably Leary-esque I'd say the fact that the vast majority of these researchers haven't experimented with these drugs themselves compromises their objectivity in a certain sense. Based on my experience and accounts from friends and acquaintances I find it likely that due to 1960s LSD hysteria doctors and researchers mis-judge 'flashbacks' as something more severe and sinister than they actually are. I don't doubt that they are a severe phenomenon, but when a doctor cites figures like '12% of infrequent users and 24% of heavy users' I'm really skeptical because 'flashbacks' are a vague and nebulous term for a field of experience that has barely been studied. From my understanding a flashback can be anything from a short lasting rush of feeling 'high' or increased sensitivity to color to a full-blown 'trip' (which I've never experienced or heard a first hand report of, but there are many points in between the two extremes I guess). It's a shame objective scientific inquiry by the mainstream of researchers in this field is hindered by decades of continuing prohibitionist rhetoric/hysteria and a scarcity of the data needed to combat the former.

zhqhqn says:

It's interesting to read people's views on flashbacks and HPPD - actually, as someone who's being diagnosed with the latter and knows one or two individuals who have experienced the former, I am certain that they are (at least phenomenologically) different. Flashbacks seem to be precipitated by stress or cannabis, and are psychologically and perceptually similar to the psychedelic experience, and tend to arise in those who have a larger dose-history, and are intense whereas HPPD is purely visually similar to drug experience (though it can entail psychological problems too), and not as intense.

I recall reading (I've lost the link) that flashbacks are uncommon among LSD research subjects, what with controls over set and setting, dosage etc. HPPD, if you have a look at David Kozin's survey on the matter, is unrelated to dose or set and setting, and can begin months or years after the actual drug use. What is astonishing is that no doctor I've ever spoken to was aware of any of this. It is a devastating condition, and I think that these two "side-effects" would rule out the use of psychedelics for treating OCD, also given the inconvenience of having to trip every 24 hours when you're already struggling with OCD. Studying the pharmacology of this OCD-reducing effect might be very useful though.

daksya says:

zhqhqn: I think that these two "side-effects" would rule out the use of psychedelics for treating OCD, also given the inconvenience of having to trip every 24 hours when you're already struggling with OCD.

I know one of the patients who is part of the case reports in the examination of LSD/psilocybin for cluster headaches (http://www.maps.org/sys/w3pb.pl?mode=search&c_pkey=22779&displayformat=allinfo&type=citation), and he isn't always tripping because tolerance rapidly develops to the psychological effects and unless you abstain for a few days, the tolerance remains.

Also during the days of legal experimentation, LSD was given daily to "severely disturbed" children (ages as low as 5-6) for between 5 to 35 months and those kids weren't in a pseudopermanent tripping state, again, because of maintained tolerance.

Haydes says:

I suffer from OCD and i am ponding on the idea of using shrooms to cure my problem. But i have not found a site on which to use and how much of it. Can someone assist me in this matter?