The appearance of the eyes serves as a direct indicator of the central nervous system’s function, reflecting physiological responses to various internal states. Substances that affect the brain’s chemical balance often trigger immediate, observable changes in ocular structures and movement. The eyes contain muscles and blood vessels innervated by the autonomic nervous system, which controls involuntary body functions. This direct connection means different substances can produce distinctly altered visual signs, from changes in the white of the eye to the size of the pupil.
The Appearance of Redness and Bloodshot Eyes
The most commonly recognized visual sign is redness or “bloodshot” eyes, which occurs when small blood vessels on the surface of the eye become enlarged. This phenomenon, known as vasodilation, involves a significant increase in blood flow to the ocular tissues. Tetrahydrocannabinol (THC) in cannabis often causes this effect by lowering blood pressure, which subsequently causes the conjunctiva vessels to dilate.
This influx of blood makes the sclera, the white part of the eye, appear visibly red or pink. Other substances, such as alcohol, also lead to vasodilation, making the eyes appear flush. Additionally, some substances reduce tear production, leading to dryness and irritation that further exacerbates the red, inflamed look.
Alterations in Pupil Size
Changes in the pupil’s diameter are among the most telling signs, as pupil size is tightly regulated by the balance of the autonomic nervous system. The iris contains two sets of muscles—the sphincter pupillae (constriction) and the dilator pupillae (opening)—whose activity is dictated by neurotransmitter levels. When the sympathetic nervous system is activated, mydriasis, or pupil dilation, occurs because the dilator muscle contracts.
This dilation is characteristic of stimulants such as cocaine, amphetamines, and hallucinogens, which increase norepinephrine release, leading to an unnaturally large pupil size. Conversely, the parasympathetic nervous system causes miosis, or pupil constriction, as the sphincter muscle contracts. Depressants, especially opioids like heroin and fentanyl, cause the parasympathetic system to dominate, resulting in pupils that appear abnormally small or “pinpoint.”
Changes in Focus and Eyelid Positioning
Beyond color and size, the quality of the gaze can change due to impaired muscle control and cognitive function. One such sign is ptosis, or the drooping of the upper eyelid, often seen with heavy central nervous system depressants like opioids and benzodiazepines. This drooping is caused by muscle relaxation affecting the levator palpebrae superioris muscle, making the eyes appear partially closed and heavy.
The gaze itself may appear “glassy” or unfocused, often resulting from reduced blinking caused by intoxication. This lack of blinking causes the eye surface to dry out, altering light reflection and giving the eyes a glazed, watery sheen. Another significant sign is nystagmus, which involves involuntary, rapid, and repetitive eye movements. Nystagmus reflects poor muscle coordination and is frequently observed with high levels of alcohol or the use of dissociative substances such as PCP and ketamine.
Important Context and Non-Drug Causes
It is important to recognize that changes in eye appearance are indicators of a physiological state, not definitive proof of substance use. Numerous common conditions can mimic the effects seen from intoxication. For example, red or bloodshot eyes are a frequent symptom of environmental allergies, fatigue, dry eye syndrome, or irritation from contact lenses.
Altered pupil size can also be caused by factors entirely unrelated to substance use. Mydriasis can be a side effect of various prescription and over-the-counter medications, including certain antidepressants, decongestants, and antihistamines. Furthermore, both extremely large and small pupils can indicate serious underlying medical conditions, such as brain injury, stroke, or glaucoma, which affect the neurological control of the iris.