Photophobia from Atropa belladonna

Photophobia can be described as an abnormal physical intolerance to the visual perception of light, manifesting as discomfort, pain, or a strong aversion to light sources that would normally be well-tolerated. Despite its name suggesting a phobia or psychological fear, photophobia is not a psychological condition but rather a genuine physical sensitivity in which exposure to light — particularly bright or harsh light — produces unpleasant sensations ranging from mild discomfort to sharp, stinging pain in the eyes.

The primary mechanism behind substance-induced photophobia is pupil dilation, also known as mydriasis. Under normal conditions, the pupils constrict in response to bright light, regulating the amount of light that enters the eye and reaches the retina. When a substance causes the pupils to dilate, this regulatory mechanism is impaired or overridden, allowing excessive amounts of light to flood the retina. The resulting overstimulation of the photoreceptive cells produces the characteristic discomfort and pain associated with photophobia.

At lower intensities, this effect manifests as a mild sensitivity to bright environments, causing one to squint or feel slightly uncomfortable in direct sunlight or under fluorescent lighting. Sunglasses may be sufficient to manage the discomfort. At higher intensities, the sensitivity can become severe enough that even moderate lighting conditions cause significant pain or distress, making it difficult to keep one's eyes open in normally lit environments. In these cases, the individual may strongly prefer dim or dark environments and may find any direct light source to be genuinely painful.

Photophobia is most commonly induced under the influence of moderate to heavy dosages of psychedelic compounds such as LSD, psilocybin, mescaline, and DMT, all of which reliably produce pronounced pupil dilation. It can also occur with stimulants like MDMA and amphetamines, which similarly dilate the pupils, and with certain dissociative compounds. The severity of the photophobia typically correlates with the degree of pupil dilation, which in turn correlates with dosage.

Photophobia is often accompanied by other coinciding effects such as pupil dilation, brightness alteration, visual acuity enhancement, and colour enhancement. Managing this effect is relatively straightforward — wearing sunglasses, dimming indoor lights, or avoiding brightly lit environments can significantly reduce discomfort. Many experienced psychedelic users routinely prepare sunglasses in advance when anticipating an experience that is likely to produce pupil dilation and associated light sensitivity.