Breathing through the mouth now and then is normal. It happens during a cold, a hard workout, or a stressful moment. The question is what happens when it becomes the main way a person breathes, day and night, for months or years.
In children especially, long-term mouth breathing is linked to tooth decay, gum problems, changes in face and jaw growth, and poor sleep. Doctors call it one of the most common harmful oral habits in children, with reported rates between 11% and 56% depending on the study and the country [1]. In adults it is less risky, though it still dries the mouth and can disturb sleep [3].
Here is what the science actually shows, where the evidence is strong, and where it is still shaky.
What counts as mouth breathing?
Mouth breathing is usually defined as more than 25% to 30% of your air passing through the mouth instead of the nose [1]. It is rarely a habit with no cause. In most people, the nose is blocked, so the body switches to the mouth as a backup route [1, 3].
Why people breathe through the mouth
The most common cause in children is enlarged adenoids and tonsils. The adenoids are a pad of immune tissue at the back of the nose, and the tonsils sit at the back of the throat. They grow fastest between ages 2 and 6, and if they grow too big, they narrow the airway and force mouth breathing [1].
Other common causes include [1, 3]:
- Allergic rhinitis, meaning an allergy that stuffs the nose. Air pollution has made this more common in many cities
- A deviated septum, where the wall inside the nose leans to one side
- Swollen nasal structures called turbinates
- Nasal polyps, small growths in the nose
- Chronic colds and sinus infections
For parents, the usual clues are a child who sleeps with the mouth open, snores, drools on the pillow, wakes with bad breath, or has chapped lips and dark circles under the eyes [3].
What it does to the teeth
This is the best-documented area, and the mechanism is simple: a stream of air dries out the mouth.
Saliva is the mouth’s shield. It washes away food, neutralizes acid, and fights bacteria. When mouth breathing dries the mouth, that shield weakens. The result, shown in studies of children and teenagers, is [1, 2]:
- More plaque, the sticky bacterial film on teeth
- More gingival bleeding, meaning gums that bleed when brushed
- Red, inflamed gums (gingivitis)
- Higher levels of cavity-causing bacteria
- More tooth decay, especially early lesions and advanced cavities
- Bad breath, called halitosis
- More teeth grinding during sleep, called bruxism, which can wear down teeth and strain the jaw joint
A 2026 systematic review of 11 studies confirmed this pattern, though it noted the evidence for cavities is stronger than the evidence linking mouth breathing to full gum disease [2]. The dry gums of mouth breathers also tend to resist healing even after dental cleaning, because the nightly drying keeps repeating [2].
For adults, the picture is milder: dry mouth, bad breath, and sore gums rather than changes in the shape of the face [3].
What it does to the face and jaw
This is where claims sometimes run ahead of the evidence, so it is worth being careful.
The classic picture. A child who mouth breathes for years tends to hold the tongue low instead of pressed against the roof of the mouth. Without that steady tongue pressure, the upper jaw can grow narrower and taller. In one study, the roof of the mouth was 11% higher in mouth-breathing children than in nose breathers. The lower jaw can rotate downward and backward, the face grows longer, the chin looks recessed, and the lips may not close fully at rest [1].
The bite changes too. A study of 1,616 children found mouth breathing linked to an open bite (front teeth that do not meet), a crossbite (upper and lower teeth that misalign sideways), and a large overjet (upper front teeth sticking out) [1]. Doctors sometimes call the full pattern “adenoid facies” or “long face syndrome” [1, 3].
The honest caveat. Not all studies agree. Some reviews found the quality of the included research was low, and others could not confirm a firm link between mouth breathing and changes in face shape [1]. Part of the problem is that there is no agreed test for diagnosing mouth breathing, so studies measure it in different ways. The animal evidence is striking, though: monkeys forced to breathe through their mouths developed longer faces and backward-rotated jaws, which suggests a real effect [1].
The fairest summary: chronic mouth breathing in growing children probably nudges the face and teeth away from typical development, especially when it comes from a blocked airway, but the exact changes vary from child to child and the research quality needs improvement [1].
What it does to sleep
This is the most important area for health.
Mouth breathing is often a sign of sleep-disordered breathing, a spectrum that runs from simple snoring to obstructive sleep apnea (OSA), where the airway repeatedly closes during sleep. In children, enlarged adenoids and tonsils are the dominant cause, and OSA affects roughly 1% to 5% of children [4].
During sleep, mouth breathing and a narrow airway can mean:
- Snoring and noisy, labored breathing
- Restless sleep and frequent waking
- Bedwetting and night terrors in some children
- Waking unrefreshed, with a dry mouth and morning headache
The daytime effects matter most. Poor-quality sleep in children shows up as irritability, hyperactivity, trouble concentrating, and behavior problems that can look like ADHD. This overlap is well documented, though mouth breathing alone has not been proven to cause ADHD [3, 4].
In adults, mouth breathing during sleep is linked to snoring and waking with a dry mouth, and it often goes together with OSA. Risk factors for adult OSA include obesity, a large neck, alcohol, smoking, and sleeping on the back [4, 5].
Can it be treated?
Yes, and this is the hopeful part. Treatment targets the cause, not the habit alone [1, 3, 4]:
- Allergy medicines and steroid nasal sprays for allergy-related blockage
- Surgery to remove enlarged adenoids or tonsils (adenotonsillectomy), the first-line treatment for children with OSA from enlarged tissue
- Septoplasty to straighten a deviated septum
- Rapid palate expansion or other orthodontic treatment to widen a narrow upper jaw in growing children
- Weight management when obesity contributes
Research suggests that removing obstructing adenoids and tonsils in growing children can normalize breathing and even partly reverse developing face and bite changes [1]. Surgery is not perfect, though. OSA persists in up to 40% of children after adenotonsillectomy, especially in children with obesity or severe disease before surgery, and some need further treatment [4].
Myths vs facts
| Claim | Evidence-based answer |
|---|---|
| “Taping the mouth shut at night fixes mouth breathing.” | Myth and potentially dangerous. There is no solid evidence for mouth taping, and taping the mouth of someone whose nose is blocked, or who has sleep apnea, can reduce airflow. Treat the blocked nose instead [3]. |
| “Mouth breathing causes ADHD.” | Unproven. Poor sleep from breathing problems can look like ADHD and may worsen attention, but a direct cause has not been shown [3, 4]. |
| “Occasional mouth breathing during a cold harms a child’s face.” | Myth. Short periods during illness or exercise are normal and harmless. Concern is about chronic, habitual mouth breathing in growing children [1, 3]. |
| “Adult mouth breathing will reshape an adult face.” | Myth. Facial bone changes are mainly a risk in childhood, while bones are still growing. In adults the issues are dry mouth, gum irritation, snoring, and sleep quality [3]. |
| “Every child who mouth breathes will get crooked teeth.” | Myth. The risk is higher, but many mouth breathers develop normal bites, and many children with crowded teeth are not mouth breathers [1]. |
| “If a child’s nose is blocked, it is just a habit they will outgrow.” | Partly myth. Sometimes true for colds, but a blocked airway from adenoids, allergies, or a septum problem will not fix itself and deserves a check [1, 3]. |
What to do if you are concerned
For a child who often mouth breathes, snores most nights, sleeps restlessly, or shows daytime tiredness and behavior changes, a first step is a visit to a doctor, dentist, or an ear, nose, and throat (ENT) specialist. Diagnosis may involve an examination of the nose and throat, a sleep study (polysomnography, an overnight recording of breathing), and a dental or orthodontic assessment [1, 4].
Do not ignore persistent snoring in a child. It is the loudest clue that breathing, sleep, and growth are not working well together [4].
Bottom line
Habitual mouth breathing is not just a quirky habit, especially in children. It dries the mouth and raises the risk of cavities and inflamed gums. In growing children it may steer face and jaw development toward a longer, narrower pattern and bite problems. And it is often a visible sign of a blocked airway and disturbed sleep, which can show up as tired, restless, or struggling children by day. The encouraging news: find and treat the blocked nose early, and most of these effects can be stopped or even reversed [1, 4].
Disclaimer: This article is for general education only and does not replace advice from a doctor, dentist, orthodontist, or ear, nose, and throat specialist. If you notice an error or would like to send feedback or corrections, please contact us.
References
- [1] The impact of mouth breathing on dentofacial development: a concise review, Frontiers in Public Health, PubMed Central
- [2] Dental caries and periodontal outcomes in mouth-breathing children and adolescents: a systematic review, PubMed
- [3] Mouth breathing: symptoms, complications and treatment, Cleveland Clinic
- [4] Apnea in children, StatPearls, NCBI Bookshelf
- [5] Sleep apnoea, NHS
