Early one night in the fall of 1987, a college freshman ate half of a microdot of lysergic acid diethylamide on his way to a party. He was young, but more than a little familiar with mind-altering chemicals: LSD, mescaline, psilocybin, and other, less common psychedelics. This trip, by comparison, turned out to be only a “mild experience.” The tingling euphoria, splendid visuals, and sudden bursts of insight mostly wore off by the time he retired to his dorm. But the following morning, some effects still remained.

“I opened my eyes to see what time it was,” he said, on the condition of anonymity. “As I looked away, I immediately realized that the light from the digital clock was streaking.” Throughout the day, other signatures of the hallucinogen high struck him. When he shifted his gaze from a page he was reading, a ghostly afterimage of the text materialized in the air, hanging legibly for a few moments. When he turned a page, a long cascading series of replicas trailed behind, like a stroboscopic photograph.

The streaking and trailing and after-imaging persisted for days. He began to panic. “I really lost it,” he said. “I was sitting in one of my first college classes and, like, hallucinating.” He met with psychologists, who could discern little. He called his parents, who could discern less. He became unhinged, wandering campus in a daze, squinting at the world as if through a kaleidoscope. “I broke down,” he said. “I could no longer go to class. I couldn’t do anything.” He quit school, moved back home, and entered rehab. His search for a diagnosis came up empty: no underlying medical condition, nor had the drug been laced with something sinister. Weeks, months, then years went by. The trip just wouldn’t end.

Psychedelic lore is littered with cautionary tales. But it remains to be seen whether reports of hallucinogen persisting perception disorder—quite literally, the persistence of hallucinogen-induced perceptions—should count among them. Hallucinogens are enjoying something of a revival: the drugs are being tried recreationally by nearly one in five American adults (approaching that of the nineteen-sixties), while being tested empirically for their powers to heal alcoholism and other addictions, anxieties from impending death, P.T.S.D., major depression, and even cluster headaches. Reading too much into H.P.P.D., some say, could squelch the renewed intrigue—even though, to some extent, the risk factors, causes, and effective treatments remain a mystery. Others, though, suspect that unraveling this mysterious disorder could reveal clues for the more familiar ones. According to Dr. Henry Abraham, a lecturer in psychiatry at Tufts University School of Medicine who privately sees patients with substance-related disorders, neurophysiological shifts observed in H.P.P.D. patients “may yield useful models for anxiety, depression, psychosis, and even addiction.”

A chronic and debilitating condition, H.P.P.D. warps the perceptual faculties: the external senses are marred by a constellation of mostly visual distortions, while the internal ones are paralyzed by a concoction of dissociative symptoms, panic attacks, and depression. The doors of perception are not so much cleansed, as Aldous Huxley famously found after his first experience on mescaline, as they are cracked open and left askew.

H.P.P.D. does not generate hallucinations, technically speaking. Sufferers can appreciate that their perceptual aberrations are unreal—that their surroundings only appear blurred by afterimages (palinopsia) and trails (akinetopsia); shimmered by sparkles and flashed by bright bolts of light; interrupted by transparent blobs of color floating around; electrified by visual snow; magnified or shrunk by “Alice-in-Wonderland” symptoms; adorned by halos around objects, around people’s heads. The pseudo-hallucinations are ultimately unconvincing, if deeply unsettling.

Eventually, a sense of permanent unreality casts a pall over the acid-fuelled dreamscape, and sufferers disassociate—from the world, due to derealization, and from themselves, due to depersonalization. At a recent Society of Biological Psychiatry conference, Dr. Abraham presented findings, later published in the S.B.P. 2012 supplement, that suggest up to sixty-five per cent of H.P.P.D patients chronically endure panic attacks, and fifty per cent, major depression. Some patients feel their only relief is suicide.

The cluster of symptoms first appeared in the Diagnostic and Statistical Manual of Mental Disorders in 1986. Ever since, the official diagnosis has been lumped together with “flashbacks.” Brief fragments of a trip that occasionally bubble up to one’s consciousness, flashbacks may arise from sudden spikes in the cerebral cortex—stirring perceptions, sensations, or emotions mimicking those of the hallucinogen high, in the absence of any chemical. But as the term has been popularized, flashback has been rendered “virtually useless” diagnostically, writes Dr. John Halpern, an assistant professor of psychiatry at Harvard Medical School and lead author of the most recent literature review of H.P.P.D. In the review, published in Drug and Alcohol Dependence, Dr. Halpern reasons that by conflating two distinct diagnoses, a strict definition of H.P.P.D. has remained elusive, leaving its prevalence obscured. Yet, “it seems inescapable,” he concludes, based on twenty related studies dating back to 1966, “that at least some individuals who have used LSD, in particular, experience persistent perceptual abnormalities reminiscent of acute intoxication, not better attributable to another medical or psychiatric condition.”

Peer-reviewed accounts of drug users whose world had been transfigured permanently can be found as early as 1983, prefiguring the initial D.S.M. entry. In a case-control study of a hundred and twenty-three LSD users, Abraham was among the first to catalogue reports from those who flashed psychedelic and never turned off: a struggling shoe salesman whose dark-brown pairs bled into the navy-blues; a confused student whose text jumbled into “alphabet soup”; a distracted office worker whose flower pot slid back and forth along the windowsill. “This isn’t flashbacks,” said Abraham. “We have to call it what it is: a persisting perception disorder.”

Preliminary estimates of the prevalence of H.P.P.D. dismissed the disorder as an outlier, implicating as few as one in fifty thousand hallucinogen users. The most recent large-scale survey, questioning nearly twenty-five hundred users, found that over one in twenty-five were considering treatment for H.P.P.D.-like symptoms. But because participants, recruited from the popular drug information Web site Erowid, did not represent the average dabbler, and because only a small portion of them had actively sought medical care, the tally remains somewhat inconclusive. “Unfortunately,” writes Halpern, assessing the scant literature, “the data do not permit us to estimate, even crudely, the prevalence of ‘strict’ H.P.P.D.”

If “strict” cases of H.P.P.D. turn up only rarely in scientific journals, though, at HPPDonline.com, a Web forum tracking research developments and connecting sufferers, nearly nine thousand monthly visitors give some indication of what lies beyond the academic purview. They report burning and throbbing and numbing and tingling. They claim that surfaces undulate (“breathing walls”), objects vanish (“they mix with the floor”), and beams of light splinter into shards of extended rays (“star-bursting”). They share encounters that seem inexplicable—”fluids flowing down from my left temple,” “a chemical aftertaste”—and plead for the group’s insight. They raise suspicions: “Every time I walk past a certain type of tree the leaves begin to shake.” They despair: “I hear my brain.”

And they may be making their symptoms worse. While H.P.P.D. sufferers do misperceive their environment, some researchers suspect that severe anxiety—perhaps an underlying condition—aggravates those misperceptions. As noted by Matthew Baggott, a postdoctoral fellow in psychiatric genetics at University of Chicago, fMRI studies generally show close links between the attention and visual systems.