Close-up of a dog's nose
on September 01, 2021

Dog Runny Nose: Causes, Colours & When to Worry

A dog's runny (or snotty) nose is usually nothing serious — but the colour of the discharge and how long it lasts are the two biggest clues to whether it's a minor irritation or something that needs a vet visit. This guide covers the real causes, what different colours of discharge actually mean, the UK-specific grass seed risk that catches many owners out, what it means for puppies specifically, and when it's genuinely time to get it checked.

Written by Jamie Sinclair, who writes about canine health, nutrition, and everyday care — focused on translating veterinary research and real ingredient science into practical, honest guidance UK dog owners can use.

Table of Contents

Nasal Discharge in Dogs: What Does It Mean?

Nasal discharge simply means fluid — watery, thick, or coloured — coming from one or both nostrils. Owners often describe it as a runny or snotty nose, and it's one of the most common signs owners notice when something's affecting a dog's nose or sinuses, ranging from a mild, short-lived sniffle to a sign of something that needs veterinary attention.

What Does the Colour of the Discharge Tell You?

The colour and consistency of the discharge is one of the most useful things you can observe before calling your vet:

Discharge type What it often suggests Urgency
Clear and watery, both nostrils Allergies, mild irritation, or early viral infection Monitor; vet if it persists beyond 24-48 hours
Thick, yellow or green Secondary infection or significant inflammation Book a vet visit
Bloody, even slightly Trauma, foreign body, or in rare cases a tumour Same-day vet call
One nostril only, sudden onset Classic pattern for a lodged foreign object (e.g. grass seed) Same-day vet call
Foul-smelling, one-sided, with pawing at the nose Often a foreign body that's been present a while, or a tooth-root infection Vet visit within a day or two

What Actually Causes a Runny Nose?

A German veterinary study that worked through 85 dogs with nasal discharge using CT imaging, rhinoscopy, and biopsy found the causes broke down as: nasal tumours (28%), oronasal defects (26%), idiopathic chronic rhinitis (20%), foreign bodies (10%), and fungal (aspergillosis) infection (8%). Strikingly, not one single case was caused by a primary bacterial infection — despite 72% of the dogs in the study having already been given antibiotics before diagnosis. That's a useful thing to know: a persistent runny nose often isn't something antibiotics alone will fix, which is exactly why getting an actual diagnosis matters more than guessing.

Common everyday causes include:

  • Seasonal allergies — pollen, dust, or mould
  • A foreign object — grass seeds are a very common culprit, especially after walks through long grass (see below)
  • Breed disposition — flat-faced (brachycephalic) breeds like Boxers, Pugs, Shih Tzus, and Bulldogs are more prone due to their airway shape
  • Dental issues — an infected tooth root can affect the nearby sinus
  • Viral or respiratory infections — including kennel cough, which typically also brings a harsh, honking cough alongside any nasal discharge
  • Canine distemper — a serious, vaccine-preventable viral illness that also causes eye discharge, coughing, and later neurological signs in unvaccinated dogs
  • Nasal tumours — more common in older, longer-nosed breeds, and the leading cause in the study above

Grass Seeds: The Most Preventable UK Cause

This deserves its own section, because it's both extremely common and genuinely preventable with the right awareness. A large-scale UK study analysed electronic health records from 245 veterinary practices, mining 4.58 million canine consultations between 2014 and 2020, and identified 1,037 confirmed grass seed foreign body cases (O'Neill et al., 2021, Preventive Veterinary Medicine). The seasonal pattern was striking: 90.6% of cases occurred between June and September, and the odds of a grass seed case in July were almost 175 times higher than in January. Grass seeds most often lodge in the paws, ears, and eyes, but the nose and airways are a well-documented site too, typically causing sudden, violent sneezing, pawing at the face, and nasal discharge from one nostril.

The practical takeaway: if your dog develops sudden, one-sided nasal discharge or repeated violent sneezing after a walk through long grass between June and September, a grass seed is a genuinely likely explanation, and it's worth mentioning to your vet specifically rather than just describing "a runny nose." A seed lodged in the nasal passage cannot be safely removed at home — it typically requires an endoscope under sedation or general anaesthetic, since the seed's barbed shape means it can migrate further into the tissue rather than working its way back out.

Why Flat-Faced Breeds Are More Prone

Flat-faced (brachycephalic) breeds — Pugs, Bulldogs, Boxers, Shih Tzus, and similar — have a shortened skull shape that compresses the same amount of nasal tissue into a much smaller space. This crowding, sometimes alongside narrowed nostrils (stenotic nares) as part of brachycephalic airway syndrome, makes normal drainage and airflow through the nose less efficient, which can produce more visible nasal discharge, snuffling, and noisy breathing even when nothing is acutely wrong. It also means these breeds may take slightly longer to clear a minor irritation than a longer-nosed dog would, and any genuinely new or worsening symptom is worth checking sooner rather than assuming it's "just how they are."

Does Your Dog's Age Matter? Puppy Nasal Discharge

Age is a genuinely useful clue. Puppy nasal discharge is more likely to be linked to something congenital, an infection, or a foreign object than in an adult dog. A puppy with a runny nose is often just picking up a mild viral bug (similar to a human cold) from other dogs, particularly around puppy classes or busy parks, and it usually clears within a few days with rest and monitoring. In older dogs, the cause is more likely to be dental disease, a tumour, or a systemic condition — which is part of why persistent nasal discharge in an older dog is always worth having properly checked rather than watched. For either age, if your puppy or older dog has a snotty nose alongside lethargy, coughing, or reduced appetite, don't wait it out — book a vet check.

What Happens at the Vet: How a Runny Nose Gets Diagnosed

Knowing what to expect can make a vet visit feel less daunting. For a straightforward, short-lived case, a physical exam and a look up each nostril with a good light is often enough. For anything persistent, one-sided, or accompanied by other symptoms, your vet may recommend:

  • Bloods — to check for signs of infection or systemic illness
  • X-rays or CT imaging — to look at the nasal cavity and sinuses in detail; CT gives a much clearer picture of bone and soft tissue than a standard X-ray
  • Rhinoscopy — a small camera passed into the nasal passage, usually under sedation or general anaesthetic, which can also be used to retrieve a foreign body like a grass seed on the same visit
  • Biopsy — taking a small tissue sample if a tumour or chronic inflammatory condition is suspected

This might sound like a lot for "just a runny nose," but as the 85-dog study above shows, the range of possible causes is wide, and several of them (tumours, fungal infection, foreign bodies) genuinely can't be distinguished from a simple sniffle without a proper look.

Other Symptoms to Watch For

  • A swollen or painful nose
  • Difficulty breathing or noisy breathing
  • Pawing at their face
  • Reduced appetite
  • Sneezing, snorting, or reverse sneezing
  • Lethargy or restlessness

How to Help

First, try to work out what's behind it. If you can see an obvious foreign object and feel confident, you can try gently removing it — otherwise, this is a job for your vet. If there's nothing obvious, and especially if the discharge has lasted over 24 hours or is coloured, crusty, or bloody, it's time for a vet assessment rather than home treatment.

In the meantime, a few things can help your dog feel more comfortable:

  • Use a humidifier — or sit with your dog in a steamy bathroom during a hot shower — to ease a stuffy nose
  • Keep them hydrated and comfortable while you wait for a vet appointment if symptoms are mild

Prevention Tips

Vaccination — staying up to date, particularly against canine distemper, is one of the most effective preventive steps. This is usually given between 8 and 16 weeks of age.

Dental care — regular teeth cleaning and dental chews help prevent the plaque buildup that can lead to tooth-root infections affecting the sinuses.

Avoiding long grass in seed season — given how sharply grass seed cases spike between June and September, this is genuinely the single most effective seasonal precaution: stick to shorter grass or paths where practical during these months, and check your dog's face, ears, and paws for seeds after every walk through longer grass.

When to See Your Vet

Veterinarian examining a dog during a routine checkup

Book a vet visit if the discharge:

  • Lasts more than 24 hours
  • Is yellow, green, crusty, or bloody
  • Comes from one nostril only, especially if sudden
  • Is accompanied by breathing difficulty, lethargy, or loss of appetite
  • Follows a walk through long grass, especially between June and September

FAQ

Why does my dog have a runny nose?
Most often it's allergies, a mild irritation, or a foreign object like a grass seed — but persistent or coloured discharge should always be checked by a vet, since research shows bacterial infection is rarely the true underlying cause.

What does the colour of my dog's nasal discharge mean?
Clear and watery often points to allergies or a viral infection; thick yellow or green can suggest a secondary infection; any blood, even a small amount, warrants a same-day vet call.

Is a runny nose in dogs an emergency?
Usually not, but bleeding, breathing difficulty, or a sudden one-sided discharge should be checked promptly rather than waited out.

Can allergies cause a runny nose in dogs?
Yes — seasonal allergies to pollen, dust, or mould are one of the most common causes of clear nasal discharge in dogs.

Are certain breeds more prone to a runny nose?
Yes — flat-faced (brachycephalic) breeds like Pugs, Bulldogs, Boxers, and Shih Tzus are more prone due to their airway shape, which compresses the same nasal tissue into a much smaller space.

Will antibiotics fix my dog's runny nose?
Not necessarily — research has found bacterial infection is rarely the actual underlying cause of persistent nasal discharge in dogs, which is why a proper diagnosis matters more than antibiotics alone.

Why does my puppy have a runny nose?
Puppy nasal discharge is often a mild viral bug picked up from other dogs, similar to a human cold, and usually clears within a few days with rest. If it's coloured, persistent, or your puppy seems unwell, get it checked promptly.

When are grass seeds most likely to cause a runny nose?
UK veterinary data shows grass seed foreign bodies spike sharply between June and September, with July odds around 175 times higher than January. A sudden, one-sided discharge after a walk through long grass in these months is a genuine red flag.

Sources

This article is for general information only and isn't a substitute for veterinary advice. If the nasal discharge is persistent, coloured, or accompanied by other symptoms, please speak to your vet.

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