Pain in one eye can stem from dozens of causes, ranging from a minor scratch on the surface to a pressure emergency inside the eye. The location and type of pain matter: a gritty, stinging sensation usually points to something on the surface, while a deep ache or throbbing often signals a problem deeper inside the eye or even in the surrounding nerves. Here’s a breakdown of the most common and most serious causes.
Surface-Level Causes
The most common reason for sudden pain in one eye is a corneal abrasion, essentially a scratch on the clear front layer of the eye. This typically happens after something gets into your eye, like dust, a fingernail, or a contact lens edge. The pain feels sharp and gritty, as if something is still stuck in there even after it’s gone. You’ll likely notice tearing, sensitivity to light, and possibly blurry vision. Most uncomplicated corneal abrasions heal within 24 to 48 hours on their own.
Dry eyes can also cause one-sided pain, especially if one eye produces less moisture than the other or is more exposed to air (common if you sleep with one eye slightly open). Contact lens problems are another frequent culprit, particularly lenses worn too long, poorly fitted, or contaminated.
Styes and chalazia cause localized pain right at the eyelid. A stye is an infected oil gland that forms a tender bump, usually near the lash line. A chalazion is a clogged oil gland that swells into a firm, sometimes painful lump farther back on the lid. Both affect one eye at a time and are generally more annoying than dangerous.
Inflammation Inside the Eye
When inflammation develops inside the eye itself, the pain tends to be deeper and more persistent than surface irritation. Uveitis, inflammation of the middle layer of the eye, is one of the more serious causes of one-sided eye pain. Symptoms include a red eye, blurred vision, significant light sensitivity, and sometimes a change in pupil shape from round to irregular. You might notice that walking into a bright room causes a spike of discomfort.
Iritis, a specific type of uveitis affecting the colored part of the eye, produces similar symptoms. Scleritis, inflammation of the white outer wall, causes a boring, deep ache that can wake you at night and sometimes radiates to the forehead or cheek. These conditions often need treatment to prevent vision damage, and they can be associated with autoimmune conditions elsewhere in the body.
Infections That Target One Eye
Keratitis is an infection or inflammation of the cornea, most commonly linked to contact lens wear, especially sleeping in lenses. It causes pain, redness, light sensitivity, and sometimes a whitish spot visible on the cornea. Unlike a simple abrasion, keratitis tends to worsen rather than improve over a day or two.
Herpes infections can also affect one eye. The herpes simplex virus (the same virus behind cold sores) can infect the cornea, causing recurring episodes of pain and blurred vision. Shingles, caused by the varicella-zoster virus, sometimes travels along the nerve that supplies the forehead and eye. The earliest sign is often pain or tingling on one side of the forehead before any rash appears. Small, painful red blisters then develop along the forehead, and if a blister appears on the tip of the nose, the risk of serious eye involvement increases significantly. The eye itself becomes achy, red, light-sensitive, and swollen.
Pink eye (conjunctivitis) is a more common and usually milder infection. It causes redness and a burning or sandy sensation, sometimes with discharge. Viral and bacterial forms often start in one eye before spreading to the other.
Pressure Buildup and Glaucoma
Normal pressure inside the eye falls between 10 and 20 mmHg. In acute angle-closure glaucoma, the drainage system of the eye suddenly blocks, and pressure spikes rapidly. This causes severe pain in one eye, often accompanied by a headache, nausea and vomiting, blurred vision, and seeing rainbow-colored halos around lights. The eye typically looks very red.
This is an emergency. Without rapid treatment, the high pressure can permanently damage the optic nerve and cause irreversible vision loss. If you develop sudden, intense eye pain with any of these accompanying symptoms, you need immediate care, not a next-day appointment.
Nerve-Related and Referred Pain
Sometimes the eye itself is fine, but pain is generated by nerves behind or around it. Optic neuritis, inflammation of the nerve that carries visual signals from the eye to the brain, causes a dull ache behind one eye that gets worse when you move the eye. Vision in that eye typically dims or blurs over hours to days. Some people also see flashing or flickering lights with eye movements. Optic neuritis can be an early sign of multiple sclerosis, though it also occurs on its own.
Cluster headaches produce some of the most intense pain a person can experience, and it centers on one eye. The pain is sharp or stabbing, located in, behind, or around the eye. A single attack lasts 15 minutes to 3 hours, though most episodes run 30 to 45 minutes. During a cluster period, attacks come daily, often several times a day. The affected eye may tear up, turn red, or develop a drooping eyelid. Migraines can also cause pain around one eye, though they tend to involve a wider area of the head and often come with nausea and light sensitivity.
How to Assess Your Symptoms
A few key questions can help you gauge how urgently you need care. First, is your vision affected? Any loss of vision, blurriness that doesn’t clear with blinking, or new floaters alongside the pain raises the urgency. Second, how fast did the pain start? A sudden onset of severe pain is more concerning than a gradually building ache. Third, what does the eye look like? Deep redness (especially in a ring around the colored part of the eye), a cloudy cornea, or an irregularly shaped pupil all suggest something beyond a simple surface irritation.
Pain that follows a scratch or obvious foreign body and feels like gritty discomfort is usually a corneal abrasion. Pain that worsens when you look around, especially with dimming vision, fits the pattern of optic neuritis. Pain accompanied by a forehead rash suggests shingles. Severe pain with nausea, halos, and a rock-hard feeling eye points toward acute glaucoma.
Situations That Need Immediate Attention
Certain combinations of symptoms require emergency care rather than a wait-and-see approach:
- Severe pain with vision loss, nausea, and halos suggests acute angle-closure glaucoma.
- Pain and redness after eye surgery or an eye injection could indicate an internal infection.
- A direct blow or penetrating injury to the eye may involve a ruptured globe. If you suspect this, avoid pressing on the eye or trying to clean it.
- Chemical exposure requires immediate flushing with clean water for at least 15 to 20 minutes before doing anything else.
- Sudden vision changes in anyone over 50, especially with scalp tenderness or jaw pain while chewing, can signal giant cell arteritis, which threatens permanent blindness if untreated.
For pain that is mild, started gradually, and comes without vision changes or severe redness, it’s reasonable to monitor for a day or two, particularly if you can identify an obvious trigger like dry air, long screen time, or a new pair of contact lenses. But any pain that worsens over 24 hours, comes with vision changes, or is intense enough to interfere with daily activities warrants a prompt evaluation.