The pupil is the dark, central opening of the eye that regulates the amount of light reaching the retina. Changes in its size are immediate, visible physiological responses to external factors, like light, and internal stimuli, such as emotional states or chemical imbalances. When a person ingests psychoactive substances, these chemicals often interfere with the body’s communication systems. This interference leads to noticeable and predictable changes in the pupil’s diameter by overriding the eye’s normal regulatory processes.
How the Autonomic Nervous System Controls Pupil Size
The diameter of the pupil is precisely controlled by two opposing sets of muscles within the iris, which are governed by the autonomic nervous system. This system manages involuntary bodily functions, such as heart rate and digestion, without conscious thought.
Pupil constriction is handled by the parasympathetic nervous system, often called the “rest and digest” branch. Its neurotransmitter, acetylcholine, signals the circular sphincter pupillae muscle to contract, making the pupil smaller.
Conversely, pupil dilation is driven by the sympathetic nervous system, known for the “fight or flight” response. This system releases norepinephrine, which stimulates the radial dilator pupillae muscle to contract, pulling the pupil open to let in more light. Psychoactive drugs alter pupil size by mimicking or blocking these natural neurotransmitters, hijacking the sympathetic or parasympathetic pathways to force dilation or constriction.
Substances That Cause Pupil Dilation (Mydriasis)
Mydriasis, the medical term for pupil dilation, occurs when a substance triggers a high state of arousal in the central nervous system. Stimulant drugs are a primary cause because they flood the body with chemicals that mimic the sympathetic nervous system’s stress response. Substances like amphetamines and cocaine increase norepinephrine and dopamine levels, creating a sustained “fight or flight” condition that forces the pupils to widen.
Hallucinogenic compounds, such as LSD and psilocybin, also cause significant pupil dilation. These substances primarily affect serotonin receptors in the brain, indirectly overactivating the sympathetic pathways controlling the eye. Additionally, anticholinergic agents, found in certain medications and plants, cause mydriasis by blocking acetylcholine receptors. This prevents acetylcholine from signaling the sphincter muscle to constrict, allowing the dilator muscle’s influence to dominate, resulting in a wide pupil.
Substances That Cause Pupil Constriction (Miosis)
The opposite effect, known as miosis, is a noticeable constriction of the pupil, sometimes resulting in “pinpoint pupils.” This effect is most famously associated with opioid substances, including prescription painkillers like oxycodone and hydrocodone, as well as heroin and fentanyl. Opioids trigger miosis by activating mu-opioid receptors in the brain, which enhances the activity of the parasympathetic nervous system.
These substances centrally disinhibit the Edinger-Westphal nucleus in the brainstem, a region that controls parasympathetic outflow to the eye. This increased parasympathetic signaling causes the sphincter pupillae muscle to contract forcefully, shrinking the pupil. Because miosis is a direct effect of activating these receptors, its presence is a significant clinical indicator of opioid use or overdose. Other substances, such as organophosphate poisons, can also cause a similar effect.
Non-Drug Factors That Affect Pupil Size
While psychoactive substances cause dramatic changes, drug use is not the only factor influencing pupil size. The most common influence is ambient light, with pupils naturally constricting in bright light and dilating in darkness. Changes in emotional state also affect pupil size; intense emotional arousal, whether from fear, excitement, or cognitive effort, can trigger a sympathetic response resulting in temporary dilation.
A variety of medical conditions and prescription medications can also alter pupil diameter. Neurological disorders like Horner’s syndrome or Adie’s pupil, or physical trauma to the eye or head, can disrupt the nervous pathways controlling the iris. Certain medications, including some antidepressants, antihistamines, and eye drops for glaucoma, may inadvertently cause dilation or constriction as a side effect. Pupil size is also known to decrease naturally with age.