Pupil Constriction and Narcotics: Understanding the Connection

Most narcotics cause pupil dilation (mydriasis), but opioids are the notable exception—they cause pinpoint pupils (miosis).

Mechanism of Opioid-Induced Miosis

1. Opioids (morphine, heroin, fentanyl, oxycodone, methadone) bind μ-opioid receptors in the Edinger-Westphal nucleus (parasympathetic).
2. This stimulates the parasympathetic nervous system → releases acetylcholine at the iris sphincter muscle.
3. Iris sphincter contracts → pupil constriction (often to ≤2 mm).

Clinical Significance

• Hallmark of opioid overdose (with respiratory depression, decreased consciousness).
• Used in toxicology screening and clinical assessment.
• Reversed by naloxone (competitive antagonist) → pupils dilate rapidly.

Other Agents Causing Miosis

Cholinergics (pilocarpine), clonidine, organophosphates, pontine hemorrhage, Horner’s syndrome.

Key Distinction

Stimulants (cocaine, amphetamines), anticholinergics, hallucinogens, and most other "narcotics" (a legal term often misused) cause dilation. "Narcotic" legally includes cocaine; pharmacologically, only opioids constrict pupils.