Red eyes are a side effect that can occur after using cannabis. Which compound of the cannabis plant turns the eyes red? Is this effect dangerous? Is it harmful to use eye drops? Does CBD also cause red eyes? Find out more about this effect and what triggers it.
Why do your eyes turn red when you smoke weed? The role of THC
The main reason why smoking or consuming cannabis causes red eyes is tetrahydrocannabinol (THC), the main psychoactive compound of cannabis or marijuana. THC dilates the blood vessels, including those in the eyes. This vasodilation increases blood flow, causing the veins on the surface of the eye to expand. That increase in blood vessel size is what we visually see as red eyes.
THC, the psychoactive compound of marijuana, is what causes eye redness.
THC's vasodilating effect doesn't only happen in the eyes — it spreads throughout the body. This dilation of blood vessels can lead to a drop in blood pressure, and in some cases that hypotension can cause dizziness or even fainting, a phenomenon known in UK slang as a “whitey” or “green-out”.
Red eyes from smoking weed: the scientific explanation
The mechanism: vasodilation and histamine
THC produces vasodilation. This expansion of blood vessels happens not just in the eyes but throughout the body, which can also reduce blood pressure. THC also lowers the pressure inside the eye, a property that has been used medicinally to treat glaucoma. This effect can intensify the redness of the eyes.
On the other hand, some studies suggest that cannabis use may also increase the release of histamine, a molecule involved in allergic responses that also contributes to the dilation of blood vessels and the reddening of the eyes.
On top of that, the effect of THC on the endocannabinoid system can reduce tear secretion, causing red eyes and increasing redness. The result is less lubrication on the surface of the eye and, therefore, a feeling of dryness.
Hydration: key to reducing eye redness
Staying hydrated can help ease the side effects of cannabis, since dehydration can worsen THC's side effects, such as dry eyes and dizziness brought on by cannabis use.
Does CBD also cause red eyes?
Industrial hemp, containing minimal THC, would not produce these effects, or at least not as noticeably, depending on individual sensitivity. To put it in perspective, marijuana-type cannabis (which contains THC) usually has a THC content ranging from 10% to 30% or higher. By contrast, industrial hemp — the source of CBD — has a THC content below 0.2%.
Which eye drops cure red eyes from smoking weed?
If you need to reduce red eyes quickly, vasoconstrictor eye drops are the most effective option. They work in a few minutes by contracting the blood vessels that THC has dilated, returning the white look to the eye. That said: use them only occasionally, because they can cause a rebound effect if overused.
How red-eye drops work
So-called “red-eye relief” drops contain an α-adrenergic active ingredient that acts locally on the blood vessels of the conjunctiva, contracting them. It is exactly the opposite effect of THC, which is why they reverse the redness. The most common active ingredients are phenylephrine, tetrahydrozoline (also called tetryzoline), naphazoline and oxymetazoline. The most well-known brands in the UK and internationally include Visine, Optrex, Clear Eyes and Murine.
The American Academy of Ophthalmology (AAO) advises against prolonged use of eye drops containing tetrahydrozoline or naphazoline because they can worsen redness over time.
Safer alternatives to vasoconstrictor eye drops
For that reason, regular use of red-eye drops isn't recommended. The safer approach is to use artificial tears (such as Optrex ActiMist, Hycosan or Hylo-Tear), which lubricate the eye without any rebound effect, apply a cold compress on the eyelids — cold naturally constricts blood vessels —, stay well hydrated by drinking water, or simply wait. The effect of THC on the blood vessels usually fades within 2–4 hours.
Conclusion: red eyes from smoking, a harmless but annoying effect
The reddening of the eyes when using cannabis is caused by the vasodilation induced by THC, which increases blood flow in the ocular vessels. This effect is generally harmless and temporary, although it can be annoying for some users.
Next time you notice red eyes after using cannabis, you'll know it's a physiological response of the body to THC. And while it can feel uncomfortable, it's a window into the complex and fascinating effect of cannabis on the human body.
Further reading
Wang MTM, Danesh-Meyer HV. Cannabinoids and the eye. Surv Ophthalmol. 2021 Mar-Apr;66(2):327-345. doi: 10.1016/j.survophthal.2020.07.002. Epub 2020 Aug 4. PMID: 32763339.
