Bloodshot eyes happen when tiny blood vessels on the surface of the eye dilate or break, making the white of the eye look pink or red. The causes range from completely harmless (a night of poor sleep) to genuinely urgent (a sudden spike in eye pressure). Most of the time, redness resolves on its own, but knowing what’s behind it helps you decide whether to wait it out or get help.
How Eye Redness Works
The white part of your eye is covered by a thin, transparent membrane packed with tiny blood vessels. Normally these vessels are so small you can’t see them. When something irritates, inflames, or physically damages the eye, those vessels widen to increase blood flow to the area. More blood in wider vessels equals visible redness. This is the same inflammatory response that makes a scraped knee turn red, just on a surface where the contrast against white tissue makes it obvious.
Dry Eyes and Screen Time
Dry eye is one of the most common reasons for persistent redness. Your tear film is a thin layer of moisture that protects and lubricates the eye’s surface. When it breaks down too quickly or isn’t produced in sufficient quantity, the exposed surface becomes irritated and the vessels dilate in response.
There are two main forms. In one, the glands that produce tears become inflamed and simply don’t make enough fluid. In the other, the oily outer layer of the tear film is deficient, so tears evaporate too fast. The two types often overlap, which is part of why dry eye can be stubborn to treat.
Screens make this worse in a surprisingly direct way. You normally blink 15 to 20 times per minute, but during focused computer or phone use that rate drops to as few as 4 to 6 blinks per minute. Each blink spreads a fresh layer of tears across the eye. Fewer blinks means the tear film dries out between refreshes, leaving the surface exposed and irritated. If your eyes consistently look red by the end of a workday, reduced blinking is a likely contributor.
Conjunctivitis (Pink Eye)
Conjunctivitis is inflammation of that same surface membrane, and the type you have determines how it feels, how long it lasts, and whether it’s contagious.
Viral conjunctivitis is the most common form. It produces watery (not thick) discharge, often starts in one eye before spreading to the other, and may come with swollen lymph nodes near the ear. It typically clears up on its own within 5 to 14 days. If caused by herpes simplex virus, it tends to stay in one eye and can produce small blister-like lesions on the eyelid.
Bacterial conjunctivitis looks different. The hallmark is thick, yellowish-green discharge that can glue your eyelids shut overnight. Itching is usually minimal. It resolves within 7 to 10 days, though antibiotic drops can speed that up. One reliable clue: if your eyelids are stuck together in the morning, there’s no itching, and you have no history of recurring eye problems, bacteria are the most likely cause.
Allergic conjunctivitis is the itchy one. The discharge is watery or stringy rather than thick, and intense itching is the dominant symptom. It affects both eyes simultaneously and doesn’t cause swollen lymph nodes. Pollen, pet dander, and dust mites are the usual triggers.
Alcohol, Cannabis, and Tobacco
Cannabis is the classic example. THC binds to receptors in the walls of blood vessels and directly triggers them to relax and widen. This vasodilation reduces resistance to blood flow, which is why it also lowers blood pressure slightly. The vessels on the eye’s surface respond the same way, filling with blood and turning visibly red. The effect is dose-dependent: more THC means redder eyes.
Alcohol causes redness through a similar but less targeted mechanism. It dilates blood vessels throughout the body, including those on the eye’s surface. It also contributes to dehydration, which reduces tear production and compounds the irritation. Tobacco smoke, meanwhile, is a direct chemical irritant. The particulates and chemicals in smoke provoke an inflammatory response on contact with the eye’s surface, whether you’re smoking or just nearby.
Broken Blood Vessels
A subconjunctival hemorrhage looks alarming: a bright red patch that can cover a large portion of the white of the eye. Unlike general redness from dilated vessels, this is actual bleeding from a tiny burst blood vessel beneath the surface membrane. Despite appearances, it’s almost always harmless.
Common triggers include heavy lifting, straining during a bowel movement, violent coughing or sneezing, vomiting, rubbing your eyes too hard, or minor eye injuries. Contact lens wearers are also more prone to these, particularly from improper lens handling. The condition is painless, doesn’t affect vision, and resolves on its own as the blood is slowly reabsorbed. Full clearing typically takes 10 to 21 days, shifting from red to yellowish-brown as it fades, much like a bruise.
Contact Lens Redness
Sleeping in contact lenses is one of the most common causes of acute eye redness in lens wearers. When you close your eyes with lenses in, bacteria that are normally harmless on the lens surface release toxins that trigger an inflammatory response in the cornea and surrounding tissue. This isn’t an active infection but rather a reaction to bacterial byproducts, and it tends to hit when you wake up: red, uncomfortable eyes that may feel gritty or watery.
The bacteria most often responsible thrive in moist environments, and having a respiratory infection like a cold or flu increases your risk because the same bacteria colonizing your throat can migrate to your lenses. Reducing overnight wear and never sleeping in lenses when you’re sick are the most effective preventive steps.
Rebound Redness From Eye Drops
Over-the-counter “get the red out” eye drops work by forcing the blood vessels on your eye’s surface to constrict. They’re effective in the short term, but using them daily for as few as 5 to 10 days can create a rebound effect. The constriction starves the tissue of oxygen, prompting it to release signals that dilate the vessels even wider once the drops wear off. Your eyes end up redder than they were before you started.
On top of that, the receptors the drops target can become less sensitive over time, meaning each dose works less well while the rebound worsens. This cycle is why these drops are meant for occasional use only. If you find yourself reaching for them daily, the underlying cause of your redness needs attention rather than masking.
Environmental Irritants
Chlorine in swimming pools, wind, dust, air pollution, strong perfumes, and low-humidity environments (airplane cabins, air-conditioned offices) all irritate the eye’s surface directly. The mechanism is straightforward: particles or chemicals contact the membrane, trigger mild inflammation, and the vessels dilate. This type of redness usually resolves within hours once you’re away from the irritant. Lubricating eye drops (the preservative-free “artificial tears” type, not the redness-reducing kind) help by flushing irritants and restoring moisture.
When Redness Signals Something Serious
Most bloodshot eyes are a nuisance, not an emergency. But certain combinations of symptoms require prompt attention.
Acute angle-closure glaucoma is a true emergency. It happens when fluid pressure inside the eye spikes rapidly, sometimes reaching three to four times normal levels. The classic combination is a suddenly red, painful eye with rapidly worsening vision. You may see colored halos around lights. The pain can radiate as a headache on the affected side, and the pressure spike can trigger nausea, vomiting, or a racing heart through a reflex connection between the eye and the vagus nerve. The pupil on the affected side may appear fixed and unresponsive to light. This condition can cause permanent vision loss within hours if untreated.
Other warning signs that warrant same-day evaluation include eye redness accompanied by significant pain (not just mild irritation), sensitivity to light, blurred or reduced vision, thick discharge in a newborn, or redness following an eye injury. Redness alone, without pain or vision changes, is rarely dangerous.