A wide range of drugs can cause mydriasis, or dilated pupils. They fall into two broad categories: drugs that stimulate the muscle that widens the pupil, and drugs that relax the muscle that constricts it. This means everything from allergy pills to cocaine to eye drops used at your optometrist’s office can trigger the same visible effect, though the underlying mechanism and duration differ significantly.
How Drugs Dilate the Pupil
Your iris has two opposing muscles. One is a ring-shaped constrictor muscle controlled by the parasympathetic nervous system, and the other is a radial dilator muscle controlled by the sympathetic nervous system. Any drug that either blocks the constrictor or activates the dilator will widen the pupil.
Drugs that block acetylcholine receptors on the constrictor muscle (anticholinergics) cause it to relax, allowing the pupil to open. Drugs that mimic adrenaline-like signals (sympathomimetics) directly contract the dilator muscle, pulling the pupil wider. Some drugs do both at once, and combining the two mechanisms is actually how eye doctors achieve the fullest dilation during an exam.
Anticholinergic Medications
Anticholinergics are the largest and most varied class of drugs that cause mydriasis. They work by blocking the parasympathetic “rest and digest” signals that normally keep the pupil relatively constricted. Many of these drugs aren’t prescribed for their pupil effects at all. Dilation is simply an unintended consequence of blocking acetylcholine elsewhere in the body.
Medications with significant anticholinergic activity include:
- First-generation antihistamines like diphenhydramine (Benadryl), used for allergies and as sleep aids
- Bladder medications like oxybutynin and tolterodine, prescribed for urinary urgency
- Anti-nausea drugs like scopolamine, often used as a patch for motion sickness
- Parkinson’s medications like benztropine and trihexyphenidyl
- Tricyclic antidepressants, an older class of antidepressants with strong anticholinergic properties
- Some antipsychotics, which carry anticholinergic effects as a secondary property
- Atropine, used both as a diagnostic eye drop and as a treatment for certain poisonings
When anticholinergic effects build up, whether from a high dose or from combining several of these drugs, the result is a recognizable pattern that emergency physicians learn with a mnemonic: “blind as a bat” (dilated pupils and blurred vision), “dry as a bone” (no sweating), “hot as a hare” (elevated temperature), “red as a beet” (flushed skin), and “mad as a hatter” (confusion and hallucinations). Dilated pupils are one of the earliest and most visible signs of this toxicity.
Stimulants and Sympathomimetics
Any drug that ramps up your sympathetic nervous system, the “fight or flight” response, can dilate your pupils by activating the radial muscle of the iris. This category includes both prescription medications and commonly abused substances.
Cocaine causes pronounced, rapid pupil dilation by preventing nerve cells from reabsorbing norepinephrine, which floods the receptors that control the dilator muscle. Amphetamines and methamphetamine work through a related but slightly different mechanism, boosting the release of norepinephrine and dopamine. MDMA (ecstasy) reliably produces wide dilation, often more noticeable than other stimulants because it also has mild hallucinogenic properties that contribute to the effect.
Prescription stimulants used for ADHD, including amphetamine-based medications and methylphenidate, can dilate pupils as well. Nasal decongestants like pseudoephedrine, phenylephrine, and oxymetazoline are also sympathomimetics. While decongestants taken orally are unlikely to cause dramatic dilation at normal doses, the mechanism is the same, and misuse or high doses can push the effect further.
Hallucinogens
Hallucinogenic drugs tend to produce some of the most pronounced and long-lasting pupil dilation. LSD causes marked mydriasis that can persist for several hours, often outlasting the peak of the psychological experience. Psilocybin mushrooms produce a similar effect through serotonin receptor activity. Marijuana also appears on clinical lists of drugs that cause dilation, though the effect is less consistent and can vary depending on the strain and dose.
Antidepressants
SSRIs and SNRIs, the most commonly prescribed antidepressants today, can cause mydriasis as a side effect. It is considered uncommon, and most people taking these medications never notice it. When it does occur, the mechanism likely involves serotonin’s indirect influence on the balance between the two iris muscles. Case reports have documented it with specific drugs like escitalopram, but it has been reported across the class. For most people the effect is mild, but it becomes relevant if you have certain eye conditions (more on that below).
Eye Drops Used for Exams
The most deliberate use of drug-induced mydriasis happens in your eye doctor’s office. The standard combination is tropicamide (which relaxes the constrictor muscle) and phenylephrine (which contracts the dilator muscle). Together they produce reliable, full dilation that allows a clear view of the retina. The effects typically last a few hours, during which bright light feels uncomfortable and near vision is blurry. Atropine drops produce a much longer-lasting dilation, sometimes persisting for a week or more, which is why it is reserved for specific clinical situations rather than routine exams.
Risk for Narrow-Angle Glaucoma
For most people, drug-induced pupil dilation is a temporary nuisance at worst. But for a small group with specific eye anatomy, it can trigger a medical emergency. Acute angle-closure glaucoma occurs when the dilated iris physically blocks the drainage channel inside the eye, causing a rapid spike in eye pressure that can damage the optic nerve.
The people most at risk are those with a shallow anterior chamber, a short eyeball, or a condition called plateau iris syndrome, where the base of the iris sits abnormally far forward. These anatomical features are more common in people who are farsighted, older adults, and women. Certain connective tissue conditions like Marfan syndrome and pseudoexfoliation syndrome also increase risk by affecting the structures that hold the lens in place.
If you have been told you have narrow angles, virtually every drug class discussed in this article carries some risk. That includes not just eye drops, but oral anticholinergics, stimulants, antidepressants with serotonergic activity, and even some sulfonamide antibiotics. The relevant classes are broad enough that it is worth mentioning your eye anatomy to any prescribing provider, not just your ophthalmologist. Drug-induced angle closure is considered an ophthalmological emergency with potentially blinding consequences if not treated quickly.
How to Tell What’s Causing It
If you notice your pupils are unusually large, the pattern of other symptoms can help identify the cause. Anticholinergic-related dilation comes with dry mouth, warm skin, difficulty urinating, and sometimes confusion. Stimulant-related dilation tends to accompany a racing heart, agitation, and elevated blood pressure. Serotonergic dilation from antidepressants is typically mild and may not come with any other noticeable symptoms.
One practical clue is whether one or both pupils are affected. Drug-induced mydriasis from something you swallowed, inhaled, or injected almost always affects both eyes equally. If only one pupil is dilated, the cause is more likely a topical exposure (accidentally touching your eye after handling certain medications or plants) or a neurological issue unrelated to drugs.