The reddening of eyes following smoke inhalation, particularly from cannabis, is a physiological response primarily attributable to the substance’s active compound, tetrahydrocannabinol (THC). THC lowers blood pressure, causing blood vessels to dilate throughout the body, including those in the eyes. This dilation increases blood flow to the ocular capillaries, making the whites of the eyes appear redder.
Understanding this physiological effect is important for both medical and social reasons. For individuals using cannabis for medicinal purposes, awareness of this side effect allows for informed decision-making regarding dosage and timing. Socially, knowledge about the cause can reduce stigma and misunderstandings associated with the visible change. Historically, observations of this phenomenon have been used, sometimes inaccurately, as indicators of substance use.