Why Does Weed Make Your Eyes Red?

by Sean Cooley
why does cannabis make eyes red

Cannabis is a plant that has been used for centuries to treat various ailments. It can be smoked, eaten in food, or drank as a tea. Cannabis contains tetrahydrocannabinol (THC), which causes the psychoactive effects of smoking weed. Smoking cannabis can cause bloodshot, red eyes because it dilates the capillaries in your eyes. Here, we will discuss seven reasons why weed can make your eyes red and what you should do about it if you smoke cannabis regularly.

1. Dilation of the Capillaries in your Eyes

Cannabis causes bloodshot eyes because of the dilation of the capillaries. The active ingredient in cannabis is THC which binds to cannabinoid receptors. There are two types of cannabinoid receptors, CB1 and CB2, both found throughout various organs in your body, including your eye. When THC activates these receptors, it can cause a quick, mild rise in blood pressure and heart rate, which in turn causes the capillaries in your eyes to dilate when blood pressure lowers back to normal.

2. Decrease in the Production of Tears

Cannabis can also cause a decrease in the production of tears by decreasing the tear-producing hormone prolactin. Prolactin is responsible for tears and saliva. When you smoke cannabis, it can cause your eyes to become dry and irritated. 

3. Fluctuations in Blood Sugar Levels

Cannabis can cause fluctuations in blood sugar levels. This is because the THC from cannabis binds to insulin receptors. When these insulin receptors are activated, the production of a hormone called glucagon is inhibited. Glucagon maintains normal blood sugar levels by stimulating glucose production and promoting its release into the blood. When glucagon is inhibited, it can cause blood sugar levels to fluctuate, making you feel various symptoms, including bloodshot eyes.

4. Pupil Dilation

Cannabis can cause pupil dilation, leading to bloodshot eyes if smoked in bright environments. Cannabis causes pupil dilation because the THC binds with receptors on nerve endings that control pupil size. This causes the pupil to dilate, increasing light exposure which can cause bloodshot eyes in bright environments like sunlight or near a window.

5. Reduction of Blinking Rate

Smoking cannabis can also cause a reduction in the blinking rate. This is because cannabis reduces the production of the neurotransmitter acetylcholine. Acetylcholine is responsible for sending signals from your brain to your muscles which control blinking. This can cause your eyes to become dry, potentially contributing to bloodshot eyes after smoking cannabis.

6. Increased Eye Inflammation

If you have recently smoked a joint, you may have experienced some mild eye irritation after smoking it. This is because the smoke from burning the rolling paper and the cannabis itself can cause inflammation in the eyes. 

7. Allergies

If you are allergic to cannabis, you may experience bloodshot or irritated eyes after smoking it. This is because cannabis contains a substance called pollen which can cause an allergic reaction. Pollen is the fine powder that is released by flowers and plants to fertilize other plants. If you have allergies, exposure to pollen can cause your eyes to become red and irritated, thus experiencing bloodshot eyes after smoking cannabis.


Although cannabis can cause bloodshot eyes, it can vary based on the level of exposure and the amount of ancillary contributing factors. If you are only smoking a joint once in a while, you may experience mild bloodshot eyes, which will usually disappear after an hour or two. If you notice that your eyes do not return to normal after a few hours, you may be experiencing something other than the harmless red eye that comes from casual cannabis smoking. If you are experiencing red-eye symptoms such increased eye inflammation for an extended period of time, you may have allergies or another condition that calls for consultation with your healthcare provider.

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