# Effects of intravenous d9-THC on pupillary reaction and pupil size: a prospective, placebo-controlled trial in healthy volunteers not regularly consuming cannabis

## TL;DR

This study shows that intravenous THC reduces the eye's response to light and makes pupils smaller in people who don't regularly use cannabis.

## Contribution

The study provides new empirical evidence on how THC affects the pupillary light reflex and pupil size in non-regular cannabis users.

## Key findings

- THC significantly reduced relative amplitude of the pupillary light reflex compared to placebo.
- THC caused a decrease in pupil size and altered constriction time and contraction amplitude.
- THC did not cause pupil dilation (mydriasis) but instead led to pupil constriction (miosis).

## Abstract

This prospective, single-blind, placebo-controlled trial examined the effects of intravenous delta-9-tetrahydrocannabinol (d9-THC) on the pupillary light reflex and pupil size in volunteers not regularly consuming cannabis.

With ethics committee approval and written informed consent, healthy cannabis-naïve or abstinent volunteers were included. Fifteen volunteers received an intravenous bolus of d9-THC and four received placebo. Pupillary reaction and pupil size were assessed by pupillography before and for 5 h after drug administration. Primary outcome was relative amplitude. Secondary outcomes were latency, velocity of contraction, constriction time, contraction amplitude, and pupil diameter.

Pupillographic measurements were significantly altered by THC: The relative amplitude was significantly reduced with a global difference between groups (p = 0.001). The relative amplitude significantly declined at 20 min after THC administration (23.5–15.0%), and stayed constant in after placebo (27.5–28.1%). Constriction time was significantly reduced with a significant global time effect (p = 0.002), global group effect (p = 0.001), and global effect of the interaction between group and time (p < 0.001). Contraction amplitude was reduced with a significant global group effect (p < 0.001). Latency and velocity of contraction demonstrated a statistically non-significant increase. Pupil size decreased after THC administration.

Pupillography can objectively detect effects of THC on the human eye. In cannabis-naïve or abstinent volunteers THC dampens the pupillary light reflex which could result in an increased sensitivity to light. THC does not cause mydriasis, but rather miosis. These results can substantiate questions regarding liability and driving ability under the influence of THC.

The study was prospectively registered at www.isrctn.com (registration number ISRCTN53019164) on 14/04/2010.