By Dr Duncan Houston
Your cat sniffs something, pauses with their mouth slightly open, then carries on as if nothing happened. That unusual face is often a normal behaviour called the flehmen response.
The useful question is what happens around the expression: a brief pause after sniffing is very different from a cat struggling to breathe or showing mouth pain.
Quick Answer
The flehmen response helps cats investigate scent signals using a specialised scent-detecting structure called the vomeronasal organ. A brief, slightly open-mouthed expression after sniffing, followed by normal behaviour, usually needs no intervention. Persistent drooling, trouble eating or repeated unexplained mouth movements need veterinary assessment. Open-mouth breathing or visible breathing effort requires immediate veterinary care.
What does the flehmen response look like?
A cat may sniff closely, lift their head, part their mouth and draw back the upper lip. They can appear to hold a surprised or sneering expression briefly before closing their mouth and resuming their activity. The Merck Veterinary Manual describes this scent-sampling posture and its role in detecting pheromones.
Pheromones are chemical signals involved in communication between animals of the same species. Flehmen gives cats another way to investigate these signals through the vomeronasal organ, also called Jacobson's organ. Urine markings and scents left by other cats are relevant examples.
The face alone does not tell you that your cat feels disgusted, anxious or aggressive. Look at the whole sequence, including what they were investigating and how they behave afterwards.
Normal scent investigation or a reason to call the vet?
| What you notice | What it suggests | What to do |
|---|---|---|
| Brief lip curl immediately after sniffing, then usual activity | Consistent with flehmen | Let your cat finish investigating a safe object. |
| New, repeated mouth movements without a clear scent trigger | The cause is uncertain | Record the context and contact your vet if it recurs. |
| Drooling, pawing at the mouth, bad breath or difficulty eating | Possible oral discomfort or illness | Contact your vet today; seek urgent care now if swallowing is impossible. |
| Open-mouth breathing, obvious effort or collapse | A breathing emergency | Go to an emergency vet now. |
A short video can help your vet distinguish a facial posture from repeated jaw movements. Only record one when your cat is comfortable and doing so will not delay care.
What else can look like a cat sneering?
Mouth or dental pain
Inflamed gums, ulcers, injuries or something caught in the mouth can change how a cat holds or moves their jaw. Drooling, reluctance to eat and pawing at the face are more concerning than the expression itself. Merck's guidance on feline mouth disorders explains why an oral examination matters. Some cats need sedation for a safe, thorough assessment.
Nausea or travel-related discomfort
Drooling and unusual mouth movements during a journey should not automatically be labelled flehmen. Motion sickness can cause nausea, salivation and vomiting in cats. Tell your vet about travel, vomiting and appetite changes; the expression alone cannot identify the cause.
Panting or breathing difficulty
Panting involves repeated breaths through an open mouth. A still photograph may hide that distinction. Asthma, heart failure and fluid around the lungs are among the conditions discussed in Cornell's guidance on feline breathing difficulty. A face shape cannot rule these out.
When is this an emergency?
Seek immediate veterinary care for open-mouth breathing, visibly laboured breathing, collapse or severe weakness. Do not wait for gum colour to change. Call the clinic while arranging transport, keep handling gentle and let your cat maintain a comfortable position. Do not force food, water or medicine into their mouth.
If apparent panting has already stopped, call your vet for advice about the episode. Do not deliberately repeat exercise or stress to see whether it happens again.
What should you do next?
- Check the context. Was there an obvious sniffing episode, and did your cat immediately return to their usual activity?
- Watch without interfering. If everything else is normal, leave your cat to investigate. There is no need to open their mouth or stop a normal flehmen response.
- Note changes. For recurrent unexplained episodes, record the trigger, approximate duration, eating changes and any drooling. Share this with your vet.
- Respond to associated signs. Contact your vet today for new persistent drooling or difficulty eating. Breathing difficulty takes priority over filming or keeping a diary.
Common mistakes to avoid
- Diagnosing anxiety or territorial conflict from one facial expression.
- Assuming an open mouth must be harmless because your cat has shown flehmen before.
- Holding scented objects against your cat's nose to provoke the response.
- Punishing scent investigation or forcing a painful mouth open.
How can you support safe scent exploration?
Normal flehmen does not need prevention. Aim to give your cat choice and a comfortable environment. The Feline Veterinary Medical Association's environmental needs advice emphasises scent, safe resting places, play and predictable interactions.
- Provide scratching areas, hiding places and familiar bedding.
- Let your cat approach safe new objects voluntarily and walk away freely.
- Avoid overpowering fragrances and strongly scented cleaning products around resting areas.
- Offer daily interactive play and rotate suitable toys, respecting your cat's interest.
- In homes with several cats, separate food, water, litter and resting resources so access is easy.
Keep routine veterinary checks, including mouth assessment, up to date. Persistent changes in eating, grooming or social behaviour deserve attention even if an occasional flehmen face looks familiar.
Frequently Asked Questions
Why does my cat open their mouth after smelling something?
They may be showing flehmen, a normal scent-sampling response. The reassuring pattern is a brief pause after sniffing followed by ordinary behaviour.
Does the flehmen face mean my cat thinks I smell bad?
No. The expression does not establish disgust or tell us exactly what information your cat has detected.
Should I stop my cat's flehmen response?
No intervention is needed when your cat is investigating something safe and otherwise comfortable. Let them choose whether to continue.
How long should a cat's flehmen response last?
It is generally brief. There is no useful stopwatch rule that proves an open mouth is harmless; the trigger, breathing and return to normal behaviour matter more.
Can flehmen cause drooling or trouble eating?
Do not use flehmen to explain persistent drooling or painful eating. Those signs warrant a veterinary assessment for oral disease or another cause.
What matters most
A brief scent-linked expression in a comfortable cat is usually simply flehmen. You do not need to interpret every facial movement, but changes in breathing, eating or comfort should guide your next step.
ASK A VET™ offers practical information to help you understand everyday feline behaviour. If your cat is struggling to breathe, seek emergency veterinary care immediately.
The ASK A VET™ Tracker logs live location and activity trends, so a change in routine never goes unnoticed.
Every ASK A VET article is written and reviewed by qualified veterinarians.
