The appearance of the eyes can visibly indicate substance use, as various compounds alter the physiological processes governing ocular function. These observable changes result from drugs interacting with the body’s nervous system and musculature. The eyes are highly sensitive to these chemical influences, often displaying rapid and distinct reactions to psychoactive substances.
The Autonomic Nervous System and Ocular Changes
The size of the pupil is controlled by two opposing sets of muscles within the iris, regulated by the Autonomic Nervous System (ANS). The ANS operates without conscious control and is divided into two primary branches: the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS).
The SNS handles the “fight or flight” response, causing the radial muscle of the iris to contract and pull the pupil open, allowing more light to enter. Conversely, the PNS governs “rest and digest” functions, causing the circular sphincter muscle to contract. This action reduces the pupil’s diameter, limiting light exposure. Drugs that mimic or block the signals of these two systems directly impact this balance, leading to noticeable changes in pupil size.
Recognizing Mydriasis (Dilated Pupils)
Mydriasis is the medical term for pupil dilation, characterized by an abnormally large pupil size unresponsive to normal light conditions. This phenomenon is an effect of substances that either directly stimulate the SNS or interfere with the PNS’s ability to constrict the pupil.
Stimulant drugs, such as cocaine, methamphetamine, and amphetamines, cause mydriasis by significantly increasing neurotransmitters like norepinephrine. Norepinephrine is a chemical messenger of the SNS, and its surge forces the iris’s dilator muscle to contract. This results in a wide-open pupil, overriding the eye’s natural light reflex.
Hallucinogens, including LSD and MDMA (ecstasy), also frequently cause pronounced mydriasis by activating the sympathetic pathway. Certain prescription medications, like anticholinergics, can induce dilation by blocking acetylcholine, the neurotransmitter responsible for PNS-driven constriction. Dilated pupils can cause light sensitivity and blurred vision.
Recognizing Miosis, Redness, and Involuntary Movements
Miosis, or pupillary constriction, is the opposite reaction and is often associated with drugs that enhance PNS activity. The most common cause of miosis is the use of opioids, including heroin, fentanyl, and prescription pain relievers like oxycodone. Opioids cause this effect by increasing the signal to the sphincter muscle. This leads to characteristic “pinpoint” pupils that can be extremely small, even in dim lighting.
Ocular redness, medically known as conjunctival hyperemia, results from vasodilation, the widening of blood vessels in the conjunctiva. Tetrahydrocannabinol (THC), the main psychoactive component in cannabis, causes this effect by binding to cannabinoid receptors and temporarily dropping blood pressure. To compensate, blood vessels, including capillaries in the eye, dilate, making them visible and giving the eye a bloodshot appearance. Alcohol also causes redness through general systemic vasodilation.
Involuntary eye movements, termed nystagmus, manifest as rapid, repetitive, and uncontrolled movement of the eyes (vertical, horizontal, or rotary). This sign is commonly associated with central nervous system depressants or dissociatives, such as high doses of alcohol, Phencyclidine (PCP), and ketamine. These drugs disrupt the brain’s ability to coordinate and stabilize eye position, leading to erratic movements.