The answer to whether your pupils dilate when you are “high” is complex, but in most common scenarios, yes, various substances can cause the pupils to visibly enlarge, a process known medically as mydriasis. Pupil size is not a conscious choice; it is regulated by involuntary muscle movements controlled by the nervous system. The change in pupil size serves as a direct reflection of how a substance is chemically affecting the body’s unconscious regulatory systems. Understanding these controls helps determine which substances cause the pupils to widen or narrow.
The Autonomic Nervous System and Pupil Control
The precise size of the pupil is maintained by a delicate balance between the two opposing branches of the autonomic nervous system (ANS). The ANS is the body’s internal control system, regulating functions like heart rate, digestion, and the size of the pupils without conscious thought. Within the iris, the colored part of the eye, two sets of muscles work against each other to adjust the pupil’s diameter.
The sympathetic nervous system, often referred to as the “fight or flight” response, is responsible for causing pupil dilation. When this system is stimulated, it releases neurotransmitters, primarily norepinephrine, which cause the radial muscles of the iris (dilator pupillae) to contract. This contraction pulls the iris open, resulting in a larger pupil.
The parasympathetic nervous system governs the “rest and digest” state and is responsible for causing the pupils to constrict, a process called miosis. This branch releases the neurotransmitter acetylcholine, which signals the circular muscles (sphincter pupillae) to contract. Drugs interfere with pupil size by either mimicking or blocking the activity of these neurotransmitters.
Substances That Cause Pupil Dilation
Substances that cause mydriasis achieve this effect by either stimulating the sympathetic nervous system or by blocking the constricting action of the parasympathetic system. Stimulants reliably cause pupil dilation by activating the sympathetic response. Drugs like cocaine, amphetamines, and methamphetamine increase the levels of neurotransmitters such as norepinephrine and dopamine. This surge mimics the body’s natural fight-or-flight response, leading to the contraction of the dilator muscles and resulting in enlarged pupils.
Hallucinogens and psychedelics also consistently cause mydriasis, often more pronounced than stimulants. Substances such as LSD, psilocybin, and MDMA affect the brain’s serotonin pathways, which influence the sympathetic nervous system’s control over the eyes. The resulting dilation can last for several hours. Many prescription medications, including some antidepressants (SSRIs) and anticholinergic agents, can also cause mydriasis as a side effect by blocking acetylcholine receptors.
The effect of cannabis on pupil size is generally inconsistent and dependent on the specific study, dose, and individual. While some research suggests that THC can cause slight dilation, others report constriction or no significant change in static pupil diameter. Acute cannabis use may reduce the speed and amount of pupillary constriction in response to light.
Substances That Cause Pupil Constriction
Pupil constriction, or miosis, results from substances that either stimulate the parasympathetic nervous system or block the sympathetic system. Opioids are the most well-known class of substances that cause miosis, resulting in the characteristic “pinpoint pupils.” Opioids, including morphine, heroin, and synthetic variants like fentanyl, activate mu-opioid receptors in the central nervous system.
This activation stimulates the parasympathetic pathway, signaling the sphincter pupillae muscle to contract, which makes the pupils extremely small. Miosis is dose-dependent and is considered a classic sign of opioid use or overdose. However, in cases of severe overdose with oxygen deprivation, the pupils may paradoxically dilate.
Other substances can also cause miosis, though less reliably or dramatically than opioids. These include high doses of alcohol, which acts as a depressant, and nicotine.