well I hate to break it to you but it is not harmless. There is a real psychiatric disorder with a diagnostic code,292.89 from the DSM-5, Hallucinogen Persisting Perception Disorder. It seems to happen mostly with LSD but other halucinogens will bring it on as well. It isn't well researched because LSD isn't well researched and not that many people out of the general population take LSD. But it occurs often enough to merit it's own psychiatric diagnosis and a couple pages in the DSM. See below. There are support groups online for people with it and it's no fun at all. It's rare but unpredictable and they do not much about it but it's real and apparently occurs in about 4% of hallucinogen users.
Diagnostic Features
The hallmark of hallucinogen persisting perception disorder is the reexperiencing, when the
individual is sober, of the perceptual disturbances that were experienced while the individual
was intoxicated with the hallucinogen (Criterion A). The symptoms may include any
perceptual perturbations, but visual disturbances tend to be predominant. Typical of the abnormal
visual perceptions are geometric hallucinations, false perceptions of movement in
the peripheral visual fields, flashes of color, intensified colors, trails of images of moving objects
(i.e., images left suspended in the path of a moving object as seen in stroboscopic photography),
perceptions of entire objects, positive afterimages (i.e., a same-colored or
complementary-colored "shadow" of an object remaining after removal of the object), halos
around objects, or misperception of images as too large (macropsia) or too small (micropsia).
Duration of the visual disturbances may be episodic or nearly continuous and must cause
clinically significant distress or impairment in social, occupational, or other important areas
of functioning (Criterion B). The disturbances may last for weeks, months, or years. Other
explanations for the disturbances (e.g., brain lesions, preexisting psychosis, seizure disorders,
migraine aura without headaches) must be ruled out (Criterion C).
Hallucinogen persisting perception disorder occurs primarily after LSD (lysergic acid
diethylamide) use, but not exclusively. There does not appear to be a strong correlation between
hallucinogen persisting perception disorder and number of occasions of hallucinogen
use, with some instances of hallucinogen persisting perception disorder occurring in
individuals with minimal exposure to hallucinogens. Some instances of hallucinogen persisting
perception disorder may be triggered by use of other substances (e.g., cannabis or
alcohol) or in adaptation to dark environments.
Associated Features Supporting Diagnosis
Reality testing remains intact in individuals with hallucinogen persisting perception disorder
(i.e., the individual is aware that the disturbance is linked to the effect of the drug).
If this is not the case, another disorder might better explain the abnormal perceptions.
Prevalence
Prevalence estimates of hallucinogen persisting perception disorder are unknown. Initial
prevalence estimates of the disorder among individuals who use hallucinogens is approximately
4.2%.
