A person’s eyes can reveal a lot about whether they’re under the influence, because different substances produce distinct and often predictable changes in pupil size, eye color, and eye movement. No single sign is definitive on its own, but knowing what to look for makes it much easier to recognize intoxication when multiple signs appear together.
Pupil Size: The Most Reliable Indicator
Normal pupils range from about 2.5 to 5.0 mm in regular room lighting, roughly the size of a small pencil eraser. That size shifts naturally with light conditions, expanding up to 8.5 mm in near-total darkness and shrinking to around 2.0 to 4.5 mm under direct light. When someone is high, their pupils often fall well outside these ranges for the lighting they’re in, or they respond to light changes sluggishly or not at all.
Stimulants like cocaine, methamphetamine, and MDMA cause noticeably dilated pupils. The pupils look large and dark, even in a well-lit room where they should be smaller. The reaction to light is also slow, meaning if you shine a flashlight toward their eyes, the pupils take longer than normal to shrink.
Hallucinogens produce a similar dilation. Both LSD and psilocybin significantly increase pupil size, and they also impair the pupil’s normal ability to contract in response to light. Psilocybin actually disrupts that light response more than LSD does. This means someone on hallucinogens may squint or seem uncomfortable in bright environments because their pupils aren’t adjusting properly.
Opioids do the opposite. Heroin, fentanyl, oxycodone, and other opioid painkillers cause pinpoint pupils, sometimes so small they’re barely visible. This constriction persists even in dim lighting where pupils should be wide open, and the pupils show little or no reaction to changes in light. Pinpoint pupils in a dark room are one of the most distinctive signs of opioid use.
Cannabis is less consistent. Pupils may be slightly dilated or completely normal, but they tend to react slowly to light.
Red, Bloodshot Eyes
Redness is one of the most widely recognized signs of cannabis use, and there’s a clear physiological reason for it. THC binds to cannabinoid receptors in the eye, triggering blood vessels to widen. This increased blood flow to the surface of the eye produces that characteristic red, bloodshot appearance. The effect is temporary and fades as the high wears off, typically within a few hours.
Alcohol also causes bloodshot eyes through a similar mechanism of blood vessel dilation, though the redness tends to be less vivid than with cannabis. Heavy drinking produces more noticeable redness, especially as the night goes on.
Glassy or Glazed-Over Eyes
A glassy, unfocused look is most commonly associated with marijuana and heavy alcohol use. The explanation is straightforward: substances that depress the central nervous system slow down automatic functions like blinking. When a person blinks less frequently, the surface of the eye dries slightly and takes on a shiny, almost reflective quality. Combined with redness, this creates the classic “stoned” look that many people recognize immediately. With alcohol, glassy eyes tend to worsen as someone drinks more, and they’re often accompanied by drooping eyelids and difficulty maintaining eye contact.
Involuntary Eye Jerking
One of the less obvious signs is nystagmus, an involuntary jerking or bouncing of the eyeball. You won’t always notice it in casual conversation, but it becomes visible when the person tries to follow a moving object with their eyes. Instead of smooth, fluid tracking (like windshield wipers gliding across a wet windshield), their eyes jerk and skip (like wipers scraping across a dry one).
Alcohol is the most common cause. It disrupts the brain’s ability to control eye muscles, producing a horizontal jerking motion that becomes more pronounced at higher levels of intoxication. This is the basis of the horizontal gaze nystagmus test used in roadside sobriety checks, where an officer moves a pen or light slowly from side to side and watches whether the eyes follow smoothly.
Other substances cause nystagmus too. PCP is particularly distinctive because it can produce horizontal, vertical, and rotational eye jerking all at once. Inhalants and other central nervous system depressants also trigger it. At high doses of alcohol, PCP, or inhalants, vertical nystagmus can appear, where the eyes bounce up and down even when looking straight ahead.
Stimulants, hallucinogens, and cannabis do not typically cause this kind of eye jerking, which makes nystagmus useful for narrowing down what someone may have taken.
Difficulty Focusing Both Eyes Together
Another sign trained evaluators look for is the inability to cross the eyes when focusing on an object brought close to the face. This is called lack of convergence, and it shows up with several substance categories. Depressants like alcohol and benzodiazepines commonly cause it, as do opioids. If you hold a finger about 12 inches from someone’s face and slowly bring it closer, an impaired person’s eyes may drift outward rather than tracking inward together.
Putting the Signs Together
No single eye sign tells the full story, but combinations are far more revealing. Someone with pinpoint pupils, no light reaction, and slightly droopy eyelids is showing a pattern consistent with opioid use. Red, glassy eyes with slightly dilated pupils and slow light reaction point toward cannabis. Dilated pupils with a normal light response alongside jerky eye movements could suggest a hallucinogen, especially at higher doses.
Here’s a quick reference for the most common substances:
- Cannabis: Red, bloodshot eyes. Glassy appearance. Pupils normal or slightly dilated, slow to react to light.
- Alcohol: Glassy, bloodshot eyes. Pupils usually normal in size. Involuntary eye jerking when following a moving object. Difficulty focusing both eyes together.
- Stimulants (cocaine, meth, MDMA): Wide, dilated pupils even in bright light. Slow pupil reaction to light. No eye jerking.
- Opioids (heroin, fentanyl, prescription painkillers): Extremely small, pinpoint pupils. Little or no reaction to light changes. Droopy, heavy-lidded appearance.
- Hallucinogens (LSD, psilocybin): Dilated pupils with impaired light response. Possible eye jerking at high doses.
- PCP: Normal pupil size, but horizontal, vertical, and rotational eye jerking. Normal light reaction.
- Inhalants: Pupils normal or slightly dilated. Involuntary eye jerking present.
When It’s Not Drugs at All
Plenty of non-drug factors cause the same eye changes, and jumping to conclusions can lead you astray. Dilated pupils happen with adrenaline, sexual arousal, recent eye exams (the drops used to examine the retina dilate pupils for hours), certain prescription medications, and even brain injuries. Some people have a condition called anisocoria, where their pupils are naturally different sizes, which can look alarming but is completely benign.
Bloodshot eyes are a hallmark of allergies, dry environments, staring at screens too long, crying, or lack of sleep. Glassy eyes can result from fatigue, fever, or dehydration. Even nystagmus has non-drug causes, including inner ear disorders and certain neurological conditions.
Context matters more than any single observation. If someone has red, glassy eyes during allergy season and otherwise seems alert and coherent, allergies are the simpler explanation. But if those same red, glassy eyes come with dilated pupils, slow speech, and the smell of smoke, the picture changes considerably. The more signs you see together, especially signs that don’t match an obvious medical or environmental explanation, the more likely substance use is involved.