Drug-induced mydriasis refers to unusually dilated (enlarged) pupils caused by substance use. Many drugs, especially stimulants and hallucinogens, trigger the body’s fight-or-flight (stress) response and make the pupils expand abnormally. [1] Dilated pupils are a physical sign often noticed in people under the influence. However, pupil size can change for other reasons too, so context matters. Below, we explore which common drugs can cause dilated pupils, how to tell them apart from other causes, and when it’s time to seek help.
- Many drugs cause dilated pupils. Stimulants (e.g., cocaine, meth, MDMA) and hallucinogens (e.g., LSD) cause mydriasis by over-activating the sympathetic nervous system.
- Some drugs do the opposite (pinpoint pupils). Opioids like heroin or oxycodone constrict the pupils instead. Extremely small “pinpoint” pupils are a hallmark of opioid use (a condition called miosis).
- Pupil size is only one clue. Dilated pupils alone don’t prove drug use – darkness or excitement can also enlarge pupils. Consider other signs, and seek medical help if you suspect overdose or serious illness.
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Pupil dilation in drug use
Pupil size is controlled by two opposing forces: the parasympathetic nervous system (rest and digest) causes pupils to constrict. In contrast, the sympathetic nervous system (fight or flight) causes them to dilate. [2] Many drugs can shift this balance. If a substance increases the sympathetic response or inhibits the eye’s constrictor muscle, the result is dilated pupils. In short, any drug that excites the nervous system or blocks the calming signals can cause pupil enlargement.
Non-drug causes of dilated pupils
Not every instance of dilated pupils is due to drugs. Environmental and emotional factors may cause pupils to dilate: pupils naturally dilate in dim light, and adrenaline from strong emotions (such as fear or excitement) can also dilate pupils. [3]
Certain medical conditions can also cause mydriasis. For example, a serious head injury or stroke can damage nerve pathways and leave one pupil abnormally enlarged. (This is a critical warning sign requiring emergency care.) In everyday medicine, doctors even use special eye drops to induce temporary dilation during eye exams. [3]
What drugs cause dilated pupils?
Several substances can trigger mydriasis (pupil dilation) by stimulating the sympathetic nervous system, altering neurotransmitters, or affecting the body’s stress response. Below are the main drug categories known to cause noticeably enlarged pupils and why they have this effect.
1. Stimulants (Cocaine, amphetamines, MDMA, methamphetamine)
Stimulants such as cocaine and methamphetamine flood the nervous system with adrenaline, dopamine, and norepinephrine. This catecholamine surge forces the sympathetic system into overdrive, producing the classic wide-eyed, dilated pupils. [4]
Behaviorally, stimulant use often leaves people with an intense, overly alert demeanor — especially visible in people who use MDMA at nightclubs.
2. Hallucinogens (LSD, psilocybin, mescaline)
Hallucinogens act mainly through serotonin (5-HT2A) receptor stimulation. While their primary effects are perceptual, this serotonin-driven pathway also raises heart rate and blood pressure, indirectly keeping the pupils abnormally large. [4]
They also trigger noradrenaline release, which heightens arousal and causes pupil dilation.
3. Other recreational drugs (Ketamine, cannabis, & "bath salts")
Various other substances can also cause pupil dilation. For instance, ketamine (a dissociative drug) leads to moderately enlarged pupils (and its cousin drug PCP does the same). [5]
Cannabis use is strongly associated with bloodshot eyes. Some reports suggest pupil changes, but evidence for dilation is inconsistent and not well-established. [4]
Additionally, synthetic stimulants, like “bath salts” (synthetic cathinones), cause dilated pupils. [4]
4. Prescription drugs (Antidepressants, benzodiazepines, stimulants for ADHD)
Additionally, prescription medications can also cause dilated pupils as a side effect. Antidepressants (for example, SSRIs, SNRIs, or tricyclic antidepressants) cause mild mydriasis (and can even blur vision due to the enlarged pupil). [6]
Benzodiazepines (anti-anxiety/sedatives) generally tend to leave pupils normal or only slightly enlarged. [4]
Prescription stimulants used for ADHD (like Adderall or Ritalin) have similar adrenaline-boosting effects as other amphetamines; therefore, during use, the pupils dilate. [4]
5. Alcohol withdrawal
During withdrawal, the body’s adrenaline system becomes overactive. Symptoms include rapid heartbeat, sweating, tremors, and noticeably enlarged pupils. [7] In contrast, during intoxication, alcohol’s sedative effect may cause only slight dilation, with sluggish pupil reactions. [4]
Dilated pupils vs. Pinpoint pupils
Different types of drugs affect pupils in opposite ways. Dilated pupils, as mentioned, are usually a sign of stimulants or hallucinogens (drugs that excite the sympathetic system). [6] In contrast, pinpointpupils (excessively constricted pupils) are most often caused by opioids like heroin or oxycodone. [6]
For example, someone overdosing on opioids will usually have tiny “pinpoint” pupils that barely react to light. On the other hand, abnormally large pupils in a person behaving erratically suggest stimulant or psychedelic use or overamping. This contrast is so reliable that paramedics often check pupil size when evaluating an unconscious patient – pinpoint pupils immediately suggest an opioid overdose (prompting treatment with naloxone). Still, factors such as ambient light or the use of multiple drugs can complicate the picture.
Dilated pupils as a warning sign (and its limitations)
Dilated pupils can indeed be a warning sign of drug use, but this indicator has limitations. Normal non-drug factors, such as darkness or extreme emotions, can also cause pupils to dilate. And not all drugs dilate the eyes (for example, opioid narcotics do the opposite and cause pinpoint pupils).
Furthermore, suppose only one pupil is dilated while the other is not. In that case, it’s more likely due to a serious medical issue that warrants emergency treatment, such as head trauma or stroke, rather than drugs. [3]
Pupil size should be considered just one clue among many factors, not absolute proof of substance use. A teenager’s pupils might be dilated from low light or natural variation, so it’s best to avoid jumping to conclusions without concrete evidence. Always consider context and additional signs and factors.
When to seek help
If you observe severe or persistent pupil abnormalities accompanied by concerning symptoms, it’s important to seek medical help. Someone unconscious with pinpoint pupils should be treated as a possible opioid overdose (call emergency services immediately and give naloxone).
Even in less acute situations, if a friend or family member frequently has unusual pupil size plus behavioral changes, consider talking to a healthcare professional first, an addiction specialist, or an addiction helpline (with their consent). Early intervention (with their consent) can prevent a situation from worsening, so don’t hesitate to reach out for guidance. It’s always better to err on the side of caution. If someone presents with dilated pupils and strange behavior, you can also call Poison Control at 1-800-222-1222 for free expert advice if you’re unsure.
