Why your eyes turn red after smoking cannabis

Why Your Eyes Turn Red After Smoking Cannabis?

The Eyes Don’t Lie – Especially When You’re High

Why cannabis causes red eyes and what to do about it

We’ve all been there, and we all know what it looks like: red eyes caused by consumption of cannabis. It’s a tell-tale sign for the practiced smoker, allowing instant recognition of fellow marijuana fans, but it is also the main giveaway for concerned parents, suspicious cops, and perceptive teachers – makes you wonder where they learned to recognise the symptoms, don’t you agree?

Since before the days of ancient Greece, the eyes have been described by poets and philosophers as ‘mirrors of the soul’. Our eyes convey a lot of information about how we feel, especially to someone who knows what to look for. This seems doubly true when we’re high: red eyes can be caused by many different things, but a good joint will certainly do the trick. In this article, we look at the causes of the phenomenon, as we try to figure out a way to work around it and perhaps hide your high from nosey onlookers…

So are red eyes harmful?

Nope, there’s no harm whatsoever in the ‘red eye’ marijuana phenomenon. We can be brief about this. Red eyes are a perfectly natural reaction of your body’s cardiovascular system to the effects of tetrahydrocannabinol (THC), which is present in just about all strains and variants of weed and hash. THC has the ability to connect with certain receptors in your nervous system, and that is how smoking a joint or eating a hash muffin affects your behaviour and biology. THC interacts with your heart and blood vessels as it does with many other parts of your body, most notably your brain. So why does your high show in your eye? Let’s expose the mechanism that is responsible for this classic stoner characteristic.


Speaking in fancy medical terms, cannabis causes red eyes through vasodilation. This is a fancy word for the widening of blood vessels and capillaries, and it is triggered by tiny muscles relaxing around the vessels themselves, allowing them to expand. This widening of the blood vessels serves to lower blood pressure, because if the heart keeps pumping blood around at the same pace, it can flow more easily through the broadened corridors of your vascular system, requiring less pressure to do so. These processes occur at the subconscious level, so we have no power over vasodilation or its opposite, which is called vasoconstriction. They are autonomous responses triggered by your nervous system, of which the so-called endocannabinoid systemis a part. As you may know or may have guessed from the name, cannabinoids present in cannabis plants can interact with this endocannabinoid system. If we get enough THC in our blood, it changes activity levels in many parts of our body. It’s just how our nerves respond to cannabis. Right after smoking a joint, people experience a brief increase in heartrate and blood pressure (through vasoconstriction), followed by a drop in heartrate and vasodilation. Now, this is when the eyes go red, because the capillaries just beneath the surface of the white in your eyeball expand. Being wider, they can contain more blood, which in this case is bright red because it carries fresh oxygen to the tissues. The tiny blood vessels in your eye will become more visible as a result, and because they are so small, anyone standing nearby will see a pinkish-red glaze where the white of your eyes used to be. Some researchers claim that more THC will cause more reddening, so you could even use your red eyes as a way to test the potency of your weed – at least in theory. So there you have it: the red is perfectly normal for smokers. Nothing unhealthy or dangerous; just a normal response beyond your control. When the high has passed, your heart rate and blood pressure return to normal, your blood vessels constrict again, and the red is gone.

Hiding Red Eyes

Since vasodilation is an autonomous process that you can’t control no matter how hard you try, it seems as if there is nothing you can do about red eyes. Fortunately, medical science has come up with a few handy tricks for any stoner aiming to keep a low profile. The first of these are chemical eye drops. They were originally developed to relieve temporary discomfort for anyone suffering from irritated eyes and allergies, but hey! Red eyes after smoking are definitely not an allergy symptom, but being super aware of your stoner look can be irritating, so this trick seems legit!

A milder solution is to use artificial teardrops, sold over the counter at any pharmacy. These are intended for people trying to get rid of dry eyes, but they soothe the tissues to such an extent that the redness will go away, even after a good smoke. Keep a bottle of this stuff handy and simply apply a few drops to your eyes. The redness will be gone in a few minutes. All you need to do now is try not to betray yourself by getting the munchies and launching an all-out assault on the fridge!

Of course, there is an easier and non-medical way to hide your red eyes, and that is a stock inventory item of any smoker: sunglasses! They’re trendy, they’re fairly inconspicuous, and they fit in any handbag. Put simply, don’t leave home without them if there is even a slight chance of smoking up ahead.

Out Of The Red

So there we go. We now know that red eyes are a perfectly natural and harmless response to THC in your system. We also know what causes it at a biological level, as well as finding a few easy ways to cover them up. Its seems as though nothing can stop us from enjoying a nice spliff without anyone noticing, so go ahead – enjoy it!