A runny nose is most often a sign of your immune system responding to a virus, an allergen, or an environmental irritant. It can also signal a sinus infection, a medication side effect, or, in rare cases, something more serious. The underlying mechanism is the same regardless of cause: something triggers the mucus-producing glands and blood vessels inside your nose to go into overdrive, flooding your nasal passages with fluid.
Why Your Nose Runs in the First Place
Your nasal lining is packed with tiny glands, blood vessels, and nerve endings. When something irritates or inflames that lining, your body releases chemical signals, primarily histamine, that kick off a chain reaction. Histamine stimulates mucous glands to secrete watery discharge by activating sensory and parasympathetic nerves. At the same time, it causes blood vessels in the nasal lining to swell and become more permeable, which is why a runny nose so often comes with congestion. This process is essentially the same whether the trigger is a cold virus or tree pollen; what differs is why the immune system hit the alarm in the first place.
Common Cold
The most frequent explanation for a runny nose is a viral upper respiratory infection. You’ll typically notice a sore throat and cough alongside the drainage, and you may run a low fever. Cold symptoms tend to build over a day or two, peak around days three to four, and resolve within seven to ten days. The mucus often starts clear and thin, then thickens as the immune response ramps up.
Allergies
Seasonal or year-round allergies are the other leading cause. Pollen, dust mites, pet dander, and mold can all trigger the same histamine cascade that a virus does. The key differences: allergies almost never cause a fever or sore throat, but they frequently cause itchy, watery eyes and sneezing fits. You may also notice puffy eyelids and dark circles under your eyes. If your runny nose returns at the same time every year or flares up in specific environments, allergies are the most likely culprit.
Flu and COVID-19
Both influenza and COVID-19 can cause a runny nose, though it tends to be a supporting symptom rather than the headliner. In a 2023 comparison of hospitalized patients, rhinorrhea was nearly three times more common in flu patients (about 19%) than in those with COVID-19 (about 7%). That said, newer COVID variants, particularly those circulating since 2023, have shifted toward upper respiratory symptoms, making a runny nose more typical than it was in earlier waves. Flu generally brings more prominent chills and fever, while COVID is more associated with cough and fatigue.
Sinus Infection
A sinus infection, or sinusitis, usually develops on the tail end of a cold. The telltale signs are facial pressure or pain (often worse on one side), thick or discolored nasal discharge, and a reduced sense of smell. The pain frequently worsens when you bend forward or when atmospheric pressure changes, like during a flight. If cold symptoms seem to improve and then worsen again after about a week, or if they persist beyond ten days without improving, a sinus infection is a strong possibility. Symptoms lasting longer than 12 weeks move into the category of chronic sinusitis, which typically needs medical evaluation.
Does Mucus Color Matter?
Many people assume that yellow or green mucus means a bacterial infection requiring antibiotics. The reality is more nuanced. Green mucus gets its color from an enzyme released by white blood cells as part of the normal immune response, not specifically from bacteria. A study in the Scandinavian Journal of Primary Health Care found that while there is a weak statistical link between colored mucus and bacterial infection, it is far too unreliable to guide treatment decisions. Green or yellow discharge is a normal feature of viral infections too. In other words, mucus color alone is not a reason to expect antibiotics.
Environmental and Food Triggers
If your nose runs when you eat spicy food, step into cold air, or smell strong perfume, you likely have a form of non-allergic rhinitis sometimes called vasomotor rhinitis. This happens because the nerves controlling blood flow and mucus production in your nose overreact to stimuli that wouldn’t bother most people. It stems from an imbalance between the two branches of the nervous system that regulate the nasal lining. Common triggers include cold or dry air, strong odors (perfume, cleaning products, cigarette smoke), alcohol, and spicy foods. Gustatory rhinitis, the type triggered specifically by eating, is its own recognized subtype.
Hormonal shifts can also cause a runny nose. Pregnancy rhinitis is well documented, and some people notice increased nasal congestion and drainage during thyroid fluctuations or menstrual cycles.
Medications That Cause a Runny Nose
Several common medications can trigger nasal symptoms as a side effect. Blood pressure drugs, including ACE inhibitors and beta blockers, are among the most frequent offenders. Medications for an enlarged prostate and certain erectile dysfunction drugs can also cause it. Aspirin and other anti-inflammatory painkillers produce nasal symptoms in some sensitive individuals, occasionally as part of a broader pattern that includes nasal polyps and asthma. If your runny nose started around the same time as a new medication, that connection is worth exploring with your prescriber.
A Runny Nose in Children
Kids get runny noses constantly, mostly from the same colds and allergies that affect adults. But in young children, one cause is unique to their age group: a foreign object stuck in the nose. Toddlers are notorious for pushing small items (beads, food, bits of tissue) into their nostrils. The classic sign is foul-smelling, thick discharge from one side only, sometimes with blood-tinged streaks. Younger children who can’t describe what happened may simply be unusually irritable. One-sided drainage with a bad smell in a toddler should prompt a same-day medical visit.
Rare but Serious: Spinal Fluid Leak
In very uncommon cases, clear, watery drainage from one side of the nose can be a sign of a cerebrospinal fluid (CSF) leak, where the fluid that cushions the brain and spinal cord drains through a small defect near the sinuses. This typically follows a head injury, surgery, or occurs spontaneously in certain individuals. The discharge is usually thin, clear, and persistent, and it may come with a metallic taste in the mouth, hearing changes, or headaches that worsen when sitting up. A CSF leak carries a risk of meningitis and needs prompt medical attention.
How Long Is Too Long?
A runny nose from a cold should be clearly improving within ten days. Allergy-related drainage will persist as long as you’re exposed to the trigger but should respond to antihistamines or avoidance. If nasal symptoms drag on for more than 12 weeks, the condition is classified as chronic rhinosinusitis and generally warrants a referral to an ear, nose, and throat specialist, particularly if initial treatments haven’t helped after about four weeks. One-sided discharge, especially if bloody or foul-smelling, deserves earlier evaluation regardless of duration.