A headache behind the left eye with nausea is most often a migraine or a cluster headache, both of which can cause one-sided pain with an upset stomach and sensitivity to light. Less often, it comes from sinus pressure, eye strain, or tension. Rarely, it signals a problem that needs urgent care, such as a sudden severe ("thunderclap") headache, a bleed, or pressure changes inside the eye. This guide explains the common causes, what makes each one distinct, and the red flags that mean you should not wait.

1. Migraine

What it is: A neurological headache disorder that often causes one-sided, throbbing pain.

Why it happens: Changes in brain activity and blood vessels, often triggered by stress, poor sleep, certain foods, hormonal shifts, or bright light.

Common symptoms: Throbbing pain behind or around one eye, nausea or vomiting, sensitivity to light and sound, and sometimes visual changes (aura) before the pain starts.

Risk factors: A personal or family history of migraine, being female, and known triggers such as caffeine changes, skipped meals, or poor sleep.

When to seek evaluation: Frequent migraines that disrupt daily life, or a change in your usual pattern, are worth discussing with a doctor.

2. Cluster Headache

What it is: A severe, one-sided headache that comes in repeated "clusters" over weeks.

Why it happens: The exact cause is not fully understood, but it involves the trigeminal nerve and a brain region tied to the body's internal clock, which is why attacks often strike at the same time each day.

Common symptoms: Intense, piercing pain around or behind one eye, often with a red or watery eye, a drooping eyelid, or a stuffy nostril on the same side. Attacks usually last 15 minutes to 3 hours and can recur daily for weeks.

Risk factors: More common in men, smokers, and people with a family history.

When to seek evaluation: A first cluster-type episode should be evaluated to confirm the diagnosis and rule out other causes.

3. Sinus Pressure and Sinusitis

What it is: Inflammation or infection of the sinuses near the eyes and forehead.

Why it happens: A cold, allergies, or an infection can block sinus drainage and build up pressure.

Common symptoms: A dull, pressure-like ache behind or under one eye that often worsens when bending forward, along with nasal congestion, thick discharge, or a low fever. Nausea can occur from post-nasal drip or from the pain itself.

Risk factors: Recent cold or allergies, nasal polyps, or a deviated septum.

When to seek evaluation: Symptoms lasting more than 10 days, a high fever, or facial swelling should be checked.

4. Eye Strain and Uncorrected Vision Problems

What it is: Fatigue of the eye muscles from prolonged close work or an uncorrected vision problem.

Why it happens: Long hours on screens, reading in poor light, or needing new glasses forces the eyes to work harder.

Common symptoms: A dull ache behind one or both eyes that builds over the day, sometimes with mild nausea, and improves with rest.

Risk factors: Heavy screen use, uncorrected refractive error, and dry eyes.

When to seek evaluation: If it happens often, an eye exam can confirm whether your prescription needs updating.

5. Tension-Type Headache

What it is: The most common type of headache, related to muscle tension and stress.

Why it happens: Stress, poor posture, clenching the jaw, or neck muscle tightness can refer pain toward the eye and temple.

Common symptoms: A dull, band-like pressure that can be felt behind one eye, usually without the nausea or light sensitivity typical of migraine, though mild queasiness can occur with a severe episode.

Risk factors: Stress, poor sleep, and long periods at a desk.

When to seek evaluation: Headaches that occur more than 15 days a month deserve a medical review.

6. Acute Angle-Closure Glaucoma

What it is: A sudden rise in pressure inside the eye that is an eye emergency.

Why it happens: Fluid inside the eye cannot drain properly, causing pressure to spike rapidly.

Common symptoms: Severe pain in and behind one eye, blurred vision, seeing halos around lights, a red eye, and nausea or vomiting.

Risk factors: Older age, farsightedness, and a family history of glaucoma.

When to seek evaluation: This needs same-day emergency eye care; vision loss can become permanent within hours.

7. Sudden, Severe, or "Worst Headache of Your Life"

What it is: A small number of headaches are caused by bleeding in or around the brain, such as a ruptured aneurysm.

Why it happens: A blood vessel in the brain leaks or ruptures, causing a sudden, severe headache that peaks within seconds to minutes.

Common symptoms: An abrupt, extremely severe headache unlike any before, often with nausea, vomiting, neck stiffness, confusion, or loss of consciousness.

Risk factors: High blood pressure, smoking, and a family history of brain aneurysms.

When to seek evaluation: This is a medical emergency; call 911 immediately.

When to Seek Medical Care