When only one eye waters and just one nostril runs at the same time, the explanation almost always traces back to the fact that the eye and the nose on each side share plumbing and nerve wiring that operate independently of the other side. A blocked drainage tube, an inflamed sinus, a headache disorder, or even a dental infection can trigger symptoms on one side while the other remains completely dry. The phenomenon feels strange, but the anatomy makes it predictable once you understand a few key connections.
The Physical Connection Between Your Eye and Your Nose
Each eye has a tiny drainage channel called the nasolacrimal duct that carries tears from the inner corner of the eye down into the nasal cavity on the same side. This is why your nose runs when you cry: the overflow tears drain straight into the nose. The connection also works in the other direction, in a sense. Congestion or inflammation in one nasal passage can back up fluid and affect the eye above it. Because each side has its own separate duct, a problem on the left does not affect the right, and vice versa. That anatomical independence is the single biggest reason one-sided symptoms are so common.
Beyond the physical tube, the two sides share nerve pathways. Parasympathetic nerves that control tear production and nasal secretion can intersect at a nerve relay station called the pterygopalatine ganglion, which sits behind the cheekbone on each side.1PubMed Central. The anatomical and functional relationship between allergic conjunctivitis and allergic rhinitis When something activates those parasympathetic nerves on one side, both the tear gland and the nasal glands on that same side ramp up secretion simultaneously. This is why a watery eye and a runny nostril so often appear as a matched pair.
Cluster Headaches and Related Headache Disorders
If your one-sided eye tearing and nasal congestion come on suddenly alongside severe pain around the eye, a group of headache conditions called trigeminal autonomic cephalgias is a likely explanation. The most well-known of these is cluster headache, which causes bouts of excruciating pain on one side of the face lasting anywhere from fifteen minutes to three hours per attack. Along with the pain, the affected side produces a cascade of autonomic symptoms: the eye reddens and waters, the nostril stuffs up or runs, and the eyelid may droop or swell.2PubMed Central. Cluster headache The other side of the face stays completely normal.
The mechanism behind these symptoms involves a reflex loop between the trigeminal nerve, which carries pain signals from the face, and the parasympathetic system. When pain activates the trigeminal nerve on one side, signals travel to a brainstem relay and then back out through the facial nerve to the pterygopalatine ganglion on the same side. From there, parasympathetic nerve fibers fan out to the tear gland, the nasal glands, and blood vessels of the face, producing tearing, nasal congestion, and facial flushing all at once.3Frontiers in Pain Research. Cluster headache: an update on clinical features, epidemiology, pathophysiology, diagnosis, and treatment This reflex is called the trigeminal-autonomic reflex, and it can be triggered even in healthy people by stimulating just one nostril mechanically.4PubMed. Vagus nerve stimulation modulates the cranial trigeminal autonomic reflex
Cluster headache is the most common of these conditions, but it has rarer cousins. SUNCT syndrome, for instance, produces very brief attacks (usually lasting about a minute, though they can range from a few seconds to a few minutes) of stabbing or burning pain around one eye, accompanied by obvious redness and tearing of that eye and stuffiness or drainage from that nostril.5PubMed. SUNCT Syndrome: diagnosis and treatment All these conditions share the same underlying reflex wiring, which is why they are grouped together.6PubMed Central. Trigeminal autonomic cephalgias People who experience repeated episodes of one-sided eye watering with nasal symptoms and facial pain, especially if the pain is severe and the episodes cluster in time, should bring this pattern to a doctor’s attention.
A Blocked Tear Duct
When the nasolacrimal duct on one side becomes partially or fully blocked, tears cannot drain from the eye into the nose as they normally would. The result is a persistently watery eye on the affected side. Because the blockage is structural and almost always limited to one duct, the other eye stays dry. In a large study of people with acquired nasolacrimal duct obstruction, the most common complaint was a continuously watery eye with discharge, reported by over half of patients.7PubMed Central. Acquired nasolacrimal duct obstruction: clinical and histological findings of 275 cases
At first glance, a blocked duct seems like it would cause only eye symptoms. But the situation can also produce nasal involvement. If the blockage traps bacteria, the tear sac behind the duct can become infected, a condition called dacryocystitis. The infected sac swells near the inner corner of the eye and can leak pus backward into the nostril, producing one-sided nasal discharge. Acute dacryocystitis can progress to cellulitis of the tissue around the eye if untreated.8PubMed. Acute dacryocystitis causing orbital cellulitis and abscess Anyone with painful swelling, redness, and discharge at the inner corner of one eye should seek medical attention promptly, as the infection can spread to deeper tissues around the orbit.
Interestingly, the structure of your nose can predispose one side to blockage. Research on patients with unilateral duct obstruction found that the duct tended to be blocked on the same side where the nasal septum was deviated. In one study, the duct was obstructed on the deviated side in 21 out of 23 patients.9PubMed. Facial asymmetry and nasal septal deviation in acquired nasolacrimal duct obstruction A separate study confirmed this pattern, finding a statistically significant association between the side of septal deviation and the side of duct obstruction.10Journal of Craniofacial Surgery. Association of Facial Asymmetry and Nasal Septal Deviation in Acquired Nasolacrimal Duct Obstruction in East Asians If you have always breathed worse through one nostril, that same side may be more vulnerable to tear drainage problems down the road.
Dental Infections That Drain Into One Sinus
This is one of the less obvious causes, and it catches many people off guard. The roots of your upper back teeth sit very close to the floor of the maxillary sinus. When one of those teeth becomes infected, or when a dental procedure disturbs the thin bone separating tooth root from sinus, bacteria can enter the sinus and trigger inflammation. Because you have two separate maxillary sinuses, one under each cheek, the infection stays on the side of the offending tooth.
The hallmark symptoms of this dental-origin sinus infection are one-sided nasal discharge that is often foul-smelling or purulent, along with cheek pain and sometimes a bad taste in the mouth.11Dentistry Review. Pathophysiology and clinical presentation of odontogenic maxillary sinusitis The condition is most often triggered by dental procedures, infections of the upper teeth, or trauma to the jaw.12PubMed Central. Odontogenic sinusitis: A review of the current literature The eye on that side can water too, because sinus inflammation irritates the surrounding nerves and can partially obstruct tear drainage. If you have one-sided nasal symptoms and recently had dental work or have a toothache on the same side, mention both to your doctor, as these conditions are frequently misdiagnosed as ordinary sinusitis and treated with antibiotics that do not address the dental source.
Allergies That Hit One Side Harder
You might assume allergies always affect both sides equally, but that is not always the case. When an allergen enters one nostril more directly, particularly if a deviated septum funnels airflow asymmetrically, the allergic response can be stronger on one side. Research on nasal allergen challenges found that when histamine was applied inside one nostril, the nasal symptoms and chemical markers of inflammation increased only at the site of the challenge, while eye symptoms like itching and watering also increased after the one-sided nasal exposure.13PubMed Central. Nasal ocular reflexes and eye symptoms in patients with allergic rhinitis In other words, a localized allergic reaction in one nostril can trigger eye symptoms on the same side through the shared nerve reflexes described earlier.
This naso-ocular reflex helps explain why some allergy sufferers notice that one eye waters more than the other, or that their nasal congestion is markedly worse on one side. It does not mean the allergy itself is one-sided. Rather, the anatomy of each individual’s nose determines how evenly or unevenly the allergen distributes, and the reflex connections amplify whatever asymmetry exists. Treating the nasal inflammation with a topical antihistamine spray can block both the nasal and the eye symptoms, since the eye watering is being driven by the nasal nerve reflex rather than by allergen landing directly on the eye’s surface.
Viral Infections on One Nerve Branch
Herpes zoster, the same virus that causes shingles on the torso, can reactivate along the trigeminal nerve and affect one side of the face. When it targets the branch that supplies the nose and forehead, the result is painful blisters and crusted sores along one side of the nose, the forehead, and sometimes the upper eyelid, while the other side is completely clear. One published case described a man in his sixties who developed painful red plaques and blisters confined entirely to the right side of his nose, forehead, and upper eyelid, along with ulcers on the right side of his palate.14BMJ. Painful nasal and oral lesions
Before the blisters appear, the affected nerve branch can cause a stinging, itchy sensation and trigger one-sided tearing and nasal discharge as the nerve becomes inflamed. The virus travels along a single nerve branch, and the trigeminal nerve splits into left and right divisions that do not cross over. That strict laterality is why shingles on the face respects the midline so precisely. When the branch supplying the tip of the nose is involved, there is a higher risk that the eye on the same side will also be affected, so doctors treat this as an urgent situation requiring antiviral medication.
Foreign Bodies and Structural Oddities
In children especially, a foreign object stuck in one nostril is a classic cause of persistent one-sided nasal discharge, often with a bad smell. Adults are not immune to this either. In one remarkable case, a patient presented with left-sided nasal congestion and bloody, purulent discharge that turned out to be caused by a glass fragment lodged near the orbit from a nasal injury thirty years earlier. The retained foreign body had led to chronic sinusitis, nasal polyps, and a fungal infection, all confined to the left side.15PubMed Central. Nasal congestion caused by long-term retention of a nasal foreign body near the orbit: A case report
A much rarer structural cause is a cerebrospinal fluid leak, where a defect in the thin bone at the skull base allows the fluid surrounding the brain to drip into the nasal cavity. This produces a clear, watery discharge from one nostril that gets worse when bending forward and often has a salty taste. One case report described a young woman with exactly this presentation: intermittent clear fluid from the right nostril, worsened by leaning forward.16Beyond Direct Leak Visualisation: Magnetic Resonance Imaging as a Surrogate Marker in Spontaneous Cerebrospinal Fluid Rhinorrhoea. Beyond Direct Leak Visualisation: Magnetic Resonance Imaging as a Surrogate Marker in Spontaneous Cerebrospinal Fluid Rhinorrhoea If your one-sided nasal drainage is persistently clear and watery rather than thick or discolored, and it worsens when you lean your head forward, bring this up with a doctor. CSF leaks can cause serious complications like meningitis if they are not repaired.
Crocodile Tears and Misdirected Nerves
After an episode of facial nerve paralysis, such as Bell’s palsy, some people develop an odd symptom: one eye tears up whenever they eat. The condition is sometimes called crocodile tears syndrome, and it happens because when the damaged facial nerve regrows, some of its fibers take a wrong turn. Fibers that were originally headed for the salivary glands end up reaching the lacrimal (tear) gland instead, so the brain’s signal to produce saliva during eating gets redirected into tear production on that side.17PubMed. Crocodile Tears Syndrome The tearing is always unilateral because only the previously damaged nerve is misdirected.
Treatment has evolved over the years. In severe cases where the tearing is socially disruptive or interferes with vision, botulinum toxin can be injected into the lacrimal gland on the affected side to reduce tear output.18Eye. Treatment of gustatory hyperlacrimation (crocodile tears) with injection of botulinum toxin into the lacrimal gland The effect is temporary and needs to be repeated, but it offers meaningful relief without surgery. If you notice that one eye tears only when you eat, and you have a history of facial paralysis, this misdirected nerve regeneration is almost certainly the cause.
When to Worry and When to Wait
Most episodes of one-sided eye watering with nasal congestion are nothing dangerous. A common cold can affect one nostril more than the other early on, and the eye on that side may water in response. Sleep position alone can cause one-sided congestion that resolves when you get up. These transient episodes do not require medical evaluation.
Certain patterns do warrant a doctor visit. Persistent clear, watery discharge from one nostril that worsens when bending forward needs to be checked for a possible CSF leak. One-sided nasal discharge that is foul-smelling, especially after dental work, should raise suspicion for odontogenic sinusitis. Severe, recurring episodes of one-sided eye redness, tearing, and facial pain in discrete attacks point toward a trigeminal autonomic headache and deserve neurological evaluation. Painful blisters on one side of the nose and forehead require urgent antiviral treatment to protect the eye. And a chronically watery eye with discharge from the inner corner should be evaluated for a blocked tear duct before the stagnant fluid becomes infected.
The asymmetry itself is not the problem. In fact, it is a diagnostic gift. Symptoms that consistently stay on one side narrow down the possible causes dramatically compared to bilateral congestion and tearing, which could be anything from a cold to seasonal allergies. Paying attention to which side is affected, what triggers the symptoms, how long episodes last, and whether you have pain can help your doctor zero in on the right explanation quickly.