Why Does Cannabis Cause Red Eyes?

A person with red eye and a cannabis plant

Red eyes are probably the most well-known side effect of smoking weed. THC consumption reduces blood pressure and expands the arteries in the eyeball. Special eye drops, available over the counter, promise relief from bloodshot eyes and judgemental looks.

Why does cannabis turn my eyes red? This is perhaps one of the most common questions asked by the average cannabis consumer when first coming into contact with the plant. Cannabis is known to have psychotropic effects and therefore influence behaviour and general frame of mind.

African-American man with dreadlocks smoking cannabis

On the other hand, red eyes are known to be the most visible indication that someone has just had a cannabis high. Meanwhile, word has spread so much that this symptom alone – irrespective of its true causes – is often associated with cannabis use.

While this phenomenon obviously has a scientific explanation, the stigma linked to it has increased over the years. Maybe this is because it may have to do with many things, all of which indicate bad health or habits:

  • Flu
  • Allergies
  • Conjunctivitis
  • Fatigue
  • Alcohol abuse
  • Even non-abusive tobacco use

But when red eyes show as a result of consuming cannabis, is it in fact a bad thing?

Why do cannabis users get red eyes?

The phenomenon of bloodshot eyes due to cannabis use is primarily the result of the main active substance in cannabis: tetrahydrocannabinol, better known as THC. There is a lot of speculation as to the reason that THC causes eyes to turn red. It may be mediated by direct contact of smoke with the eyes, thus irritating the conjunctiva of the eye. However, tobacco smokers do not experience red eyes, so it’s unclear why cannabis smoking would cause red eyes in this fashion. The conjunctiva may also be aggravated by smoke entering the sinuses, thus causing red eyes this way.

Blood pressure drop against the white background

Another hypothesis is that THC affects blood pressure, causing it drop. This results in blood vessels expanding, and blood circulation throughout the body increases. The drop in blood pressure causes the arteries in the eyeball to expand, so the blood is able to circulate more visibly than normal. However, this is inconclusive, as the relationship between THC and blood pressure is complex, and doesn’t affect everybody the same way.
Whilst this side-effect of smoking weed is undesirable for most consumers, it can be beneficial for patients with painful eye conditions such as glaucoma. There is evidence that they feel relief from their symptoms as a result.

While associated with smoking in general, red eyes may also occur irrespective of the consumption method. Vaporizing cannabis or the consumption of food products containing cannabis also have the potential to cause red eyes. This is because THC, one of the main cannabinoids in cannabis, is always activated when the substance is heated to a specific minimum temperature. This is what happens when vaporizing or burning, but also when baking a cookie.

THC spelled on a board surrounded with cannabis flowers

 It is also worth noting that there are people in whom the same symptom occurs for different reasons; smoking generally causes red eyes in these individuals, regardless of whether the smoke originates from cannabis or from another substance.

What remedies are available for bloodshot eyes?

From the perspective of an all-round consumer and activist, there’s no reason to conceal red eyes caused by cannabis use. Yet because cannabis isn’t fully accepted either legally or socially, noticeably red eyes will understandably pose a problem for many people.

Woman holding cucumber slices in front of her eyes

There is no easy, immediately effective treatment method for red eyes, however. Whereas using one of the available emollients may cause symptoms to disappear entirely in one person, in someone else it may have no effect whatsoever. However, vessel-widening effects can generally be alleviated as follows:

If you need perfect, snow-white eyeballs all-year-round for whatever reason, particular varieties that contain different proportions of cannabinoids and terpenoids may be more suitable, especially in relation to red eyes. As ever, we recommend giving different things a go to see what works.

Are red eyes dangerous?

As long as your red eyes don’t stop you from doing what you need to do every day, you can rest assured that redness doesn’t generally harm your eyes, your brain or your general health. If you become a regular user – for medical or other reasons – your body will become used to the symptom to a certain extent, and the phenomenon will be relatively temporary. However, if you consistently have persistent red eyes that do not seem to go away, there may be an underlying cause that should be addressed by your physician.

  • Disclaimer:
    This article is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with your doctor or other licensed medical professional. Do not delay seeking medical advice or disregard medical advice due to something you have read on this website.


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    Sensi Seeds

    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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    Sanjai Sinha

    Dr Sanjai Sinha is an academic faculty member at Weill Cornell Medicine in New York. He spends his time seeing patients, teaching residents and medical students, and doing health services research. He enjoys patient education and practicing evidence-based medicine. His strong interest in medical review comes from these passions.
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