The familiar red and glassy-eyed stare of someone who’s high was thought to be due to the irritation of eyes by pot smoke. Now we know that weed makes your eyes red for the same reason it makes you dizzy- vasodilation.
Marijuana’s has a lot of active ingredients. Tetrahydrocannabinol (THC) is only one of the many (>113) cannabinoids present in cannabis. These compounds interact with cannabinoid receptors, which are part of the endocannabinoid system. They’re found throughout your body, notably, in your eyes.
Cannabinoids bind to cannabinoid receptors and induce the dilation, or widening, of the blood vessels. This increases the blood flow to these areas,and causes an overall decrease in blood pressure. The increased blood flow to your eyeball causes the red appearance, and the lowered blood pressure causes the dizziness.
You can test it yourself, by consuming marijuana through a non-smoked method,and looking for reddening of your eyes.