The pupil is the dark, central opening in the iris that controls the amount of light entering the eye. Its function is similar to the aperture of a camera, automatically adjusting in size to optimize vision in various lighting conditions and to protect the retina from bright light. The size of this opening is managed by two sets of tiny muscles within the iris. When certain substances are introduced into the body, they can involuntarily override this normal regulation, causing visible, physiological alterations in pupil size.
How the Nervous System Controls Pupil Size
Pupil size is tightly regulated by the Autonomic Nervous System (ANS), the involuntary control system of the body. The ANS is divided into two opposing branches that act on the two muscles of the iris. The Sympathetic Nervous System governs the “fight or flight” response, and its activation causes the radial dilator pupillae muscle to contract, which increases the pupil’s diameter, a process known as mydriasis. Conversely, the Parasympathetic Nervous System manages the “rest and digest” state, and it controls the circular sphincter pupillae muscle. When the parasympathetic system is stimulated, this muscle contracts, causing the pupil to narrow, an action called miosis. Psychoactive substances interfere with the neurotransmitters that communicate signals along these two pathways, shifting the delicate balance.
Substances That Cause Pupil Dilation (Mydriasis)
Mydriasis, or pupil dilation, is commonly associated with substances that activate the sympathetic nervous system, mimicking a heightened state of arousal. Stimulants, such as cocaine, amphetamines, and MDMA (ecstasy), reliably cause this effect by increasing the activity of neurotransmitters like norepinephrine and dopamine in the central nervous system. Norepinephrine acts on the dilator muscle to promote widening, effectively triggering a sustained “fight or flight” response.
Psychedelic substances, including LSD and psilocybin, also produce marked pupil dilation. These hallucinogens often exert their effects by interacting with serotonin receptors in the brain, which can indirectly lead to an increase in sympathetic outflow to the eyes. This type of drug-induced dilation can be quite pronounced, causing the dark center of the eye to appear much larger than normal.
In typical indoor or moderately dim lighting, a person’s pupils might measure between 3 and 5 millimeters, but substances causing mydriasis can push this size far past the upper limit of normal variation. Since the pupil is already dilated, it may show a sluggish or absent response when exposed to bright light, which is a key indicator of altered function. This large pupil size can lead to light sensitivity, or photophobia.
Substances That Cause Pupil Constriction (Miosis)
The opposite effect, miosis, is a dramatic constriction of the pupils often resulting in what are termed “pinpoint pupils”. This distinct physical sign is most notably associated with the use of opioids, which include both illicit drugs like heroin and powerful prescription medications such as fentanyl, oxycodone, and morphine. Opioids bind to mu-opioid receptors in the brain, which in turn enhances the activity of the parasympathetic nervous system. The resulting overstimulation of the parasympathetic pathway causes the iris sphincter muscle to contract forcefully and continuously, making the pupils extremely small and unresponsive to changes in light. The degree of constriction is often directly related to the dose of the opioid, making it a critical sign in cases of potential overdose.