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What Causes Bloodshot Eyes?

By H. Lo
Updated May 17, 2024
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Bloodshot eyes, a condition in which the white part of the eyes become red, occur because blood vessels in the eyes dilate or swell. As there are a variety of causes of this dilation and swelling, the condition itself is a pretty common one. Allergies, infections and trauma can cause bloodshot eyes, as well as certain other medical conditions. Any of these causes can be mild or severe, but the amount of redness in the eyes does not necessarily determine the severity of a person’s overall condition. More important symptoms to look at are accompanying pain, or vision difficulties.

Allergies can definitely be the cause of bloodshot eyes. Drugs, environmental factors and animals are a few things that can cause a person to have an allergic reaction. For example, a penicillin shot can cause a person to exhibit eye redness, in addition to other allergy symptoms. Pollen is an example of an environmental factor that can also lead to redness in the eyes. Also, certain animals can trigger an allergic reaction as well, such as a bite from a particular insect.

The list of infections and inflammations that can cause bloodshot eyes is varied and wide. While this makes it impossible to discuss them all, a few examples include blepharitis, conjunctivitis and uveitis. With blepharitis, an infection occurs in an eyelash follicle because of skin bacteria. Conjunctivitis, also known as pink eye, is an inflammation due to bacterial or viral causes. Uveitis is the inflammation of the choroid, ciliary body, and iris. In addition to eye redness, an infection can also cause the eyes to be itchy or painful and might even lead to vision problems.

Trauma to the eye can also be responsible for bloodshot eyes. This can be anything from coughing or straining, to eye surgery, to foreign objects in the eye. Coughing and straining can actually cause a condition known as subconjuctival hemorrhage, which is when a blood vessel breaks and causes one spot of redness in the white part of the eye; this minor bleeding usually goes away on its own within a couple weeks time. Bleeding in the eye can also cause bloodshot eyes and can explain the redness if a person just had eye surgery. It is understandable that foreign objects, like contact lenses, can cause red eyes because they can scratch the cornea.

Bloodshot eyes can also occur as a result of other medical conditions. A few examples are dehydration, dry eyes or eye fatigue. In addition, other conditions that can result in bloodshot eyes include acute glaucoma, keratoconus and pterygium.

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Discussion Comments

By browncoat — On Sep 20, 2014

@MrsPramm - Wearing any kind of contacts irritates my eyes so I tend to just go without them and wear glasses instead. Other than that I only ever get red eyes if I'm having an allergic reaction. First they start watering, and then they go a bit red, and then, if I don't get away from whatever is causing it, they can even start to swell up, which is horrible.

By MrsPramm — On Sep 19, 2014

@pastanaga - It's actually worse than just making your eyes bloodshot. I've heard of people going blind from leaving their contacts in overnight too often. It's not just that you can grow new capillaries, it's also that the lens itself can shrink from becoming too dry and cause little injuries on your eye that can get infected. Also, if the new blood vessels grow in the wrong place they can interfere with your sight as well.

There are some special lenses that you can wear even when sleeping, but I wouldn't even try that if you've got sensitive or dry eyes.

By pastanaga — On Sep 18, 2014

One of the things I was told when I first started wearing contact lenses was that I had to be careful to take them out each night or I would end up with permanently red eyes. From what I remember, the idea was that the eye receives its oxygen from the inside of the lid during the night and that if there was something blocking it, then it would start growing extra capillaries on the surface of the eye in order to compensate.

I don't know if this is true, or if it was more that they didn't want people to get an infection from leaving contacts in too long. But I was definitely motivated by the thought of having permanently bloodshot eyes and always remembered to take my contacts out before bed.

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