When you get a sudden runny nose, the cause is usually a virus (a cold) or a flare-up of allergies. The clues below can help you tell them apart, which makes it easier to know what to expect and how to treat it.

Clues that point to a virus

If you have been around small children or people who are sick, a virus is the most likely cause. Colds don't all feel the same — there are hundreds of different cold viruses. Some give you more stuffiness, some more sore throat, and some more runny nose.

Colds often start in one spot — one side of the nose, the throat, or the chest — and over a day or two spread to the rest of the upper airway. People often make a lot of clear, runny mucus at first.

When you aren't sure, it is safest to treat a sudden stuffy nose as if it is a cold. Assume you can spread it, and wash your hands often. If allergies might also be involved, I usually still treat it as a cold but add an antihistamine (allergy pill). When the main problem is a constantly running nose, the older "drying" antihistamines can give some relief.

Clues that point to allergies or nose irritation

If you have just been around something you are allergic to, a flare-up is possible. A short-term reaction to an irritant (vasomotor rhinitis — a sensitive nose lining that reacts to things like smells or temperature changes) can act the same way. Both usually settle down quickly once you get away from the trigger.

Common allergy triggers
Hay fever — sudden pollen blooms
Mold spores — cleaning damp areas, gardening in wet compost or mulch
A big dose of dust mites
Pet dander
Common irritants for a sensitive nose
Perfumes, solvent fumes, smoke, dust

Allergies and a sensitive nose usually come with a history of similar short episodes in the past. Sneezing, itchy red eyes, no sore throat, and no cough are clues that one of these is to blame. If sudden symptoms aren't usual for you, and there is no obvious trigger, then allergies probably aren't the reason.

When a runny nose is the main problem and has been going on for a long time, a few common causes come to mind.

Allergies can cause a long-running nose. Year-round allergies (called perennial allergic rhinitis) can come from things like dust mites, mold spores, pet dander, and cockroach particles. This kind of allergy can also depend on where you are — people with indoor year-round allergies may feel better outside or in a different building.

A sensitive, irritated nose without a true allergy (non-allergic rhinitis) can also cause mostly drainage. With this, the nose acts like it is allergic or infected, but tests don't show an allergy or infection. It usually responds best to steroid nose sprays and, in some cases, anti-fungal rinses. A reaction to mold spores in the nose lining may play a role in both this problem and long-lasting sinus infections — this is an active area of research.

One type of non-allergic rhinitis is called vasomotor rhinitis. The nose lining reacts to things like strong smells, changes in air temperature, fumes, or smoke.

Treatment for a long-running runny nose can start with over-the-counter allergy pills and allergy sprays (not decongestant sprays). Some prescription sprays also help. The best prescription spray for a runny nose is called Atrovent Nasal 0.03% — it is the only drying spray made for this. It is very safe; the most common side effect is too much dryness, which goes away quickly and can be avoided by using a smaller spray next time or by not sniffing it back hard. Atrovent doesn't do much for stuffiness.

If stuffiness is also a big part of the problem, Astelin spray or steroid nose sprays may be more helpful.

A clear, watery drip from the nose (especially from one side only) that starts after a serious head injury or nose surgery should be reported to your doctor. In rare cases, it can be a sign that the fluid that surrounds the brain (spinal fluid) is leaking into the nose. The bone that separates the nose from the brain is surprisingly thin.

In some people who have had sinus surgery — especially with older methods — mucus can pool at the bottom of the cheek sinus (maxillary sinus) and later come out suddenly in a small amount.  It is usually thick, but can be thin and watery.

Post-nasal drip (PND) that just started can come from several causes. A new cold (viral infection) can start with that drip feeling, but it usually moves on to a runny nose or stuffy nose. Allergy flare-ups can cause it too, but again there are usually other, stronger symptoms. Sometimes a mild bacterial infection causes only the drip feeling — these usually clear on their own, though some hang around.

A flare-up of acid reflux (stomach acid coming back up the throat) can also feel like post-nasal drip. This surprises many people, because they expect heartburn or indigestion to come with it — but most of the time those don't show up. The connection between post-nasal drip and a special kind of reflux that affects the throat (called LPR, or laryngo-pharyngeal reflux) is covered in more detail below.

If post-nasal drip is your only symptom and it just started, I would try: saltwater rinses, non-drying allergy pills (like Claritin), Prilosec OTC 20–40 mg in the morning and evening, and a steroid nose spray if you have one. If things get worse or are still there after 8–10 days, see a doctor for help with the diagnosis.

If post-nasal drip has been going on for a long time and is the main problem, a few causes come to mind.

Coming from the nose or sinuses

Sometimes one or more of the deep sinuses in the back gets a long-lasting bacterial infection. This can also happen in the small patch of tissue at the very back of the nose (the adenoid). The drainage from this kind of infection irritates the throat and usually leaves a noticeable red patch at the back of the throat that a doctor can see during an exam. If you fit this pattern, see a specialist.

If a long-lasting infection is suspected, the first step is usually a long course of broad antibiotics and steroid sprays or pills. If allergies are suspected, allergy pills or steroid nose sprays are tried.

If the problem still seems to come from the sinuses but the cause isn't clear, a CT scan of the sinuses is often the next step.

Coming from somewhere other than the nose

Most people think of long-lasting infections and allergies as the main causes of post-nasal drip. But when the drip is the only symptom and there isn't much else going on with the nose, I think the most common cause is actually LPR (laryngo-pharyngeal reflux). LPR is a kind of acid reflux that mostly affects the area around the voice box.

People often have a hard time believing this at first — usually because they haven't heard of it before, and because they don't have heartburn or indigestion.

LPR can show up in several ways, and not everyone has the same symptoms. Surprisingly, heartburn and indigestion are usually not part of it.


Common signs of LPR

A feeling of thick mucus in the throat
A feeling of a lump in the throat when you swallow, especially when swallowing without food or drink
Hoarseness, or a voice that gets tired easily
A long-lasting cough or constant throat clearing
A long-lasting sore throat, low down near the voice box
Waking up choking or with a burning feeling
More likely if you are overweight, snore loudly, or have sleep apnea


What to do if this sounds like you

If the problem is severe or getting worse, see a specialist. They can use a small camera to look at your voice box. This rules out the rare possibility of a throat cancer and can help confirm or rule out LPR. If LPR seems likely, the next "test" is to try strong acid-reducing medicine for a while and see if you feel better. If a long-lasting infection seems more likely, the doctor may pick a specific antibiotic, or take a swab from the back of the nose to find out what bacteria is there. If things still don't improve, a CT scan may help.


If you want to try treating it yourself first, I would start with saltwater nose rinses and high doses of antacids — not just any antacid. LPR is known to be hard to treat, and results can be slow. Get Prilosec OTC® and take 2 in the morning and 2 in the evening. Try this for at least 2 weeks, and ideally 4 weeks, before deciding if it works. This medicine is expensive, even the over-the-counter version.

Acid-blocking pills called proton pump inhibitors (PPIs), like Prilosec, have a good safety record overall, but recent studies raise some questions about taking them long term. The standard prescription dose is the same as 2 of the over-the-counter pills, taken once a day. Taking it twice a day is considered "off-label" use, but it is often needed early on to control LPR.   Prilosec and similar pills should be taken about 30 minutes before a meal.  They don't work as well at other times.

The first month on these medicines is really a test. Treatment is usually started on an educated guess. One of the best ways to confirm LPR is to see whether acid-blocking pills help. It is best to start with the strongest medicines at higher doses. Once symptoms calm down, a lower dose is usually enough to keep them away.