Can allergies cause bad breath?

Allergic rhinitis is inflammation inside the nose triggered by things like pollen, dust mites or pet dander. The NHS lists sneezing, an itchy or blocked or runny nose, itchy watery eyes, a cough and an itchy roof of the mouth as common symptoms. Bad breath is not on that list, but allergies set up three conditions that are well known to cause it.

1. Post nasal drip

Cleveland Clinic names allergies as one of the most frequent causes of postnasal drip, and lists bad breath as a symptom of postnasal drip. Mucus settles on the back of the tongue, where bacteria break it down. More on this in post nasal drip bad breath.

2. Mouth breathing

A blocked nose pushes you to breathe through your mouth, especially at night. Cleveland Clinic lists allergies among the causes of mouth breathing and bad breath and dry mouth among its symptoms. See mouth breathing and bad breath.

3. Dry mouth from allergy medicines

Cleveland Clinic says dry mouth is most often a side effect of medicines, naming antihistamines and decongestants first. Less saliva means less natural rinsing of the tongue and teeth.

Direct research on allergies and bad breath is limited. A 2016 study in an ear, nose and throat journal measured breath gases with a gas chromatograph and found that people with allergic rhinitis had higher levels of two odor compounds, methyl mercaptan and dimethyl sulfide, than people without allergies. The authors concluded allergic rhinitis is likely to result in bad breath and called for more research. It was a single small study, so treat it as supportive rather than settled.

• Control the allergy. The NHS suggests avoiding your triggers where possible and asking a pharmacist about antihistamines, steroid nasal sprays and salt water nose rinses.
• Use decongestant sprays briefly. The NHS says not to use them for longer than 5 days, because they can make a blocked nose worse. They should not be used by children under 6.
• Clean your tongue daily. Mucus and dryness both thicken the coating at the back of the tongue, so this step matters more in allergy season.
• Fight dry mouth. Sip water through the day, chew sugar-free gum and run a humidifier at night if your mouth is dry on waking.
• Brush twice and clean between your teeth daily, as you normally would.

If you suspect an antihistamine is drying your mouth, do not just stop it; ask a pharmacist or doctor whether a different option suits you.

If bad breath continues outside allergy season, or after your nose is clear, look at other causes such as gum disease, tonsil stones or reflux. Thick colored mucus with facial pain and fever points more toward a sinus infection. The NHS advises seeing a GP if your allergy symptoms get worse, affect your sleep, or you have asthma that is getting worse.