IV THC Causes Pupil Constriction and Dampened Light Reflexes, Not Dilation
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?Would regular cannabis users show similar pupillary changes or develop tolerance?
- ?Could pupillography be practical for roadside THC impairment assessment?
Trust & Context
- Key Stat:
- THC causes miosis (constriction), not mydriasis (dilation)
- Evidence Grade:
- Placebo-controlled trial with objective pupillographic measurements, limited by small sample and single-blind design.
- Study Age:
- 2025 publication (registered 2010)
- Original Title:
- Effects of intravenous d9-THC on pupillary reaction and pupil size: a prospective, placebo-controlled trial in healthy volunteers not regularly consuming cannabis.
- Published In:
- BMC ophthalmology, 25(1), 286 (2025)
- Authors:
- Kleine-Brueggeney, Maren, Priemer, Fritz, Konietschke, Frank, Theiler, Lorenz, Greif, Robert
- Database ID:
- RTHC-06843
Evidence Hierarchy
Frequently Asked Questions
Does THC dilate your pupils?
No. This controlled study found THC actually constricts pupils (miosis), contrary to common belief. It also dampened the pupillary light reflex, which could increase sensitivity to bright light.
How long does THC affect your eyes?
In this study, THC effects on pupillary function persisted for the full 5-hour observation period. The dampened light reflex and pupil changes have implications for driving and visual performance.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-06843APA
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
MLA
Kleine-Brueggeney, Maren, et al. "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, 2025. https://doi.org/10.1186/s12886-025-04107-7
RethinkTHC
RethinkTHC Research Database. "Effects of intravenous d9-THC on pupillary reaction and pupi..." RTHC-06843. Retrieved from https://rethinkthc.com/research/kleine-brueggeney-2025-effects-of-intravenous-d9thc
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.