Why does my breath smell like mothballs?

Mothballs smell of naphthalene or a similar chemical, a sharp, tarry odor. There is no published research linking a naphthalene-like breath smell to a particular illness, and none of the major references we checked list it. What we do know is that the odors of ordinary bad breath are a cocktail of compounds, and different people name the same mix differently: rotten eggs, sewage, cabbage, or mothballs.

So the honest answer is to treat "mothball breath" as a strong case of ordinary halitosis until proven otherwise, and to look for the usual sources first.

• Tongue coating. The Merck Manual explains that bacteria on the back of the tongue break down food and cells into volatile sulfur compounds such as hydrogen sulfide and methyl mercaptan. A thick coating gives a stronger, sharper smell.
• Postnasal drip and sinus problems. Cleveland Clinic lists bad breath among symptoms of postnasal drip, and the NHS lists it among sinusitis symptoms. Mucus feeds bacteria at the back of the throat, and congestion pushes you to breathe through your mouth.
• Tonsil stones. Small calcified lumps of debris and bacteria in the tonsils. Cleveland Clinic calls bad breath their main symptom, often with a bad taste.
• Dry mouth. Less saliva means less rinsing. The NHS lists dehydration, medicines, mouth breathing at night and anxiety as common causes.
• Gum disease and tooth decay. Bacteria in deep gum pockets and cavities add to the mix.

Cleveland Clinic points out it is hard to judge your own breath by cupping a hand over your mouth, so ask someone you trust. Then look for patterns:

Our bad breath self-check walks through these questions.

The NHS recommends cleaning your tongue once a day with a scraper or cleaner, brushing teeth and gums twice a day for two minutes, cleaning between teeth daily, and using sugar-free gum after strong foods. Add plenty of water, treat any congestion, and avoid smoking. If the smell is not clearly better after a few weeks, the NHS advises seeing a dentist, who can check gums, cavities and tonsils and refer you to an ENT doctor or GP if the source is elsewhere.

Be realistic about the evidence: a 2019 Cochrane review found just two small trials (46 people) of tongue cleaning versus no cleaning and rated that evidence very low certainty; evidence for most other bad-breath treatments was low to very low certainty too. Tongue cleaning is cheap and low risk, and clinicians widely recommend it, but it is one part of a routine, not a cure on its own.

Mothballs and some toilet deodorizer blocks contain naphthalene, which damages red blood cells. MedlinePlus notes stomach symptoms may not appear until two days after exposure, and later symptoms can include confusion, fast heart rate, dark or bloody urine, breathlessness and yellow skin. People with G6PD deficiency are more vulnerable. If a child or adult has eaten a mothball, call Poison Help (1-800-222-1222 in the US) or emergency services even if they seem well.