The question of whether the eyes dilate when a person is under the influence of a substance depends entirely on the chemical properties of the substance involved. The eyes can either widen (mydriasis) or narrow (miosis), depending on how the drug interacts with the body’s involuntary control systems. The normal reflex, where pupils constrict in bright light and dilate in darkness, is often overridden by the powerful effects of various psychoactive compounds. This alteration of pupil size is a direct physical manifestation of the drug’s effect on the central nervous system.
The Physiology Behind Pupil Size Changes
The size of the pupil is governed by two opposing sets of smooth muscle fibers within the iris, controlled by the Autonomic Nervous System (ANS). The sphincter pupillae muscle is arranged circularly, and its contraction causes the pupil to constrict (miosis). This constricting action is regulated by the Parasympathetic Nervous System, associated with a “rest and digest” state.
In contrast, the dilator pupillae muscle fibers radiate outward. Their contraction pulls the iris open to increase pupil size, causing mydriasis. This dilation is controlled by the Sympathetic Nervous System, which manages the body’s “fight or flight” response. The final size of the pupil reflects the balance between the constricting force of the Parasympathetic system and the dilating force of the Sympathetic system.
The Parasympathetic response is mediated by acetylcholine, which acts on the sphincter muscle to promote constriction. The Sympathetic response is primarily driven by norepinephrine, which stimulates dilation via alpha-1 adrenergic receptors on the dilator muscle. Drugs that mimic, block, or enhance the activity of these neurotransmitters or their respective nervous systems directly alter this balance and change the pupil’s diameter. A drug’s effect on pupil size provides a direct window into its mechanism of action on the ANS pathways.
How Different Substances Affect Pupil Size
Dilation (Mydriasis)
Substances categorized as stimulants and hallucinogens reliably cause mydriasis by artificially activating the Sympathetic Nervous System. Stimulants (e.g., cocaine, amphetamines, and MDMA) cause dilation by dramatically increasing norepinephrine and dopamine levels. This increased concentration of neurotransmitters overstimulates the dilator pupillae muscle, overriding the Parasympathetic tone and forcing the pupil to widen.
Hallucinogens, such as LSD and psilocybin, also cause pronounced dilation, primarily through their interaction with serotonin receptors in the brain. This activity indirectly stimulates the Sympathetic outflow to the eye, resulting in a large pupil size that is often sustained throughout the experience. This drug-induced dilation can push the pupil diameter beyond its typical range, sometimes reaching up to 9 millimeters.
Constriction (Miosis)
The class of drugs known as depressants, most notably opioids (e.g., heroin, fentanyl, and morphine), produces the opposite effect, causing miosis, or “pinpoint pupils.” These substances activate the Parasympathetic Nervous System by stimulating specific receptors in the midbrain’s Edinger-Westphal nucleus, the control center for pupil constriction. This excitation causes the sphincter pupillae muscle to contract strongly, reducing the pupil to an unusually small size, even in dim lighting.
Cannabis
The effect of cannabis (THC) on pupil size is more nuanced and often contradicts popular belief. THC does not typically cause significant, sustained dilation like stimulants or hallucinogens. Some controlled studies suggest that THC can actually cause mild miosis, or pupil constriction, in non-regular users.
More commonly, cannabis dampens the pupillary light reflex, meaning the pupils react more slowly and less intensely to changes in light. This blunted reflex can make the pupils appear slightly larger than they should be when exposed to bright light, but this is a failure to constrict properly rather than true drug-induced dilation. The most visible ocular effect of cannabis is the reddening of the conjunctiva (bloodshot eyes), caused by the drug’s vasodilatory effects.
The Duration of Pupil Changes and Related Eye Effects
The duration of drug-induced pupil changes generally mirrors the active time of the substance in the body, which varies significantly by drug and dose. Dilation caused by stimulants can persist for several hours. Mydriasis resulting from hallucinogens like LSD or MDMA is often prolonged, lasting as long as 12 to 24 hours. Conversely, the miosis associated with opioids, such as heroin, typically begins within minutes and can last for three to five hours.
Changes in pupil size are often accompanied by other observable ocular symptoms that can signal intoxication. Nystagmus (involuntary, rapid, and repetitive eye movements) is commonly seen with the use of amphetamines and ketamine, and sometimes with high doses of opioids. Substances that interfere with the ANS can also disrupt tear production, leading to dry eye syndrome or eye irritation. The visible reddening of the eyes (conjunctival redness) is a direct physiological effect of cannabis, alcohol, and cocaine use because these substances cause the blood vessels in the eyes to expand.