The effect of THC on the pupillary reaction

The effect of THC on the pupillary reaction is discussed controversially. In some police training manuals wide and sluggish pupils are described as reliable signs of the influence of THC. Some authors, who measured pupil size during THC influence observed reduced pupillary diameters. Only when a non-physiological dosage was administered in rats, a dose-related mydriasis could be detected. The aim of the present study was to characterise the effect of THC on the human pupil. 7 healthy volunteers inhaled a joint containing 40 mg hashish within the following 10 minutes. The experiments were carried out before the THC consumption as well as 40 min. and 80 min. after smoking. Pupil size and the pupillary light reflex after a short light stimulus (4 different intensities) were registered by means of the JR-videopupillography device CIP 8.00 (AMTech, Weinheim). The latency of pupillary constriction remained unchanged by THC. When compared with the sober state, a reduced pupil diameter could be observed during THC influence. Also the amplitude of pupillary constriction after the light stimulus was slightly lower under the influence of THC. In accordance with Hepler et. al (3) effects caused by THC, such as reduced amplitude of pupillary constriction, increased heart rate and reduced lacrimation, as well as salivation can be characterised as atropine-like or parasympatholytic effects.