Contrary to common belief, THC caused pupil constriction (miosis) rather than dilation in cannabis-naive volunteers, along with a significantly dampened pupillary light reflex lasting several hours.
Law enforcement, traffic safety researchers, ophthalmologists, cannabis impairment researchers
THC causes miosis (constriction), not mydriasis (dilation)
What the researchers found
Intravenous THC significantly reduced pupillary relative amplitude (from 23.5% to 15.0% at 20 minutes, p = 0.001), constriction time (p = 0.002), and contraction amplitude (p < 0.001) compared to placebo. THC caused miosis (pupil constriction), not mydriasis (dilation). Effects persisted for 5 hours.
Why it matters
This study provides objective evidence of how THC affects visual function, with implications for driving safety and impairment assessment. The dampened light reflex could increase light sensitivity and affect vision in variable lighting conditions.
The numbers in context
15 THC recipients, 4 placebo; relative amplitude decreased from 23.5% to 15.0% at 20 min; effects on constriction time (p = 0.002) and amplitude (p < 0.001); effects lasted 5 hours.
How the study worked
Prospective, single-blind, placebo-controlled trial with 15 volunteers receiving IV THC and 4 receiving placebo. Pupillary measurements taken by pupillography before and for 5 hours after administration. Cannabis-naive or abstinent participants.
What this study cannot tell us
Small sample (15 THC, 4 placebo). IV administration does not reflect typical consumption methods. Cannabis-naive subjects may respond differently than regular users. Single-blind rather than double-blind design.
How to read the evidence
Placebo-controlled trial with objective pupillographic measurements, limited by small sample and single-blind design.
When this study was published
2025 publication (registered 2010)
The bigger picture
Pupillography could serve as an objective measure of THC impairment for roadside or clinical assessments. The finding that THC causes miosis, not mydriasis, corrects a common misconception and has practical implications for impairment detection.
Questions still open
- Would regular cannabis users show similar pupillary changes or develop tolerance?
- Could pupillography be practical for roadside THC impairment assessment?
Common questions
Does THC dilate your pupils?
How long does THC affect your eyes?
Read the original research
Effects of intravenous d9-THC on pupillary reaction and pupil size: a prospective, placebo-controlled trial in healthy volunteers not regularly consuming cannabis.
BMC ophthalmology, 25(1), 286
Citation
Kleine-Brueggeney, Maren; Priemer, Fritz; Konietschke, Frank; Theiler, Lorenz; Greif, Robert. (2025). Effects of intravenous d9-THC on pupillary reaction and pupil size: a prospective, placebo-controlled trial in healthy volunteers not regularly consuming cannabis.. BMC ophthalmology, 25(1), 286. https://doi.org/10.1186/s12886-025-04107-7