The human eye often displays noticeable alterations when an individual is under the influence of various substances. These changes are not merely superficial; they represent deeper physiological shifts within the body, particularly involving the nervous system. Understanding these ocular manifestations provides insight into the broader impact of substance use.
Common Physical Changes to the Eyes
When substances affect the body, several physical changes can become apparent in the eyes. Pupil size can either enlarge (dilate) or shrink (constrict). Redness, often called “bloodshot eyes,” is common, where the white part of the eye appears discolored due to expanded blood vessels. Eyelids may also exhibit noticeable drooping, sometimes called ptosis. Involuntary, rapid eye movements, known as nystagmus, can occur, along with dry eyes or excessive watering.
The Science Behind Eye Alterations
These eye changes stem from how substances interact with the body’s autonomic nervous system, which controls involuntary functions. The sympathetic nervous system, responsible for “fight or flight,” triggers pupil dilation (mydriasis) by contracting the iris’s radial muscles. Conversely, the parasympathetic nervous system, associated with “rest and digest,” causes pupil constriction (miosis) by contracting the iris’s circular muscles. Neurotransmitters like norepinephrine and acetylcholine play a significant role; substances can alter their levels or block receptors, affecting pupil size. Redness occurs when substances cause vasodilation, expanding tiny blood vessels in the conjunctiva, the clear membrane covering the eye’s white; dry eyes can result from decreased tear production, often due to anticholinergic effects.
Substance-Specific Eye Effects
Different substances induce distinct eye changes due to their unique pharmacological actions. Cannabis commonly leads to red, bloodshot eyes because its active compounds cause vasodilation, increasing blood flow; some studies also suggest it can cause pupil dilation, while others report conflicting findings. Stimulants like cocaine, methamphetamine, and MDMA (Ecstasy) typically cause significant pupil dilation by increasing neurotransmitter release or inhibiting reuptake, activating the sympathetic nervous system. Conversely, depressants and opioids, including heroin, oxycodone, and morphine, lead to constricted or “pinpoint” pupils by activating the parasympathetic nervous system, causing the iris sphincter muscle to contract. Hallucinogens such as LSD and psilocybin often result in pupil dilation by interacting with serotonin receptors, and some, along with dissociatives like PCP and ketamine, can also induce nystagmus. Alcohol can cause bloodshot eyes and may also lead to nystagmus and drooping eyelids, particularly with heavy consumption.
How Vision Itself Can Change
Beyond visible eye alterations, substances can also affect how a person perceives the world. Blurred vision is common, as substances can impair the eye’s ability to focus or slow communication between the eyes and brain; depth perception may also be altered, making it challenging to accurately judge distances. Increased light sensitivity can occur, causing discomfort in normally lit environments, especially when pupils are dilated and allow more light to enter. Visual distortions are another potential effect, manifesting as halos around lights, tracers, or altered color perception, and individuals might also experience minor visual hallucinations, such as seeing patterns or objects not actually present. These perceptual changes are subjective, varying significantly depending on the substance, dosage, and individual physiology.