Key takeaways

  • Most persistent bad breath (halitosis) comes from bacteria in the mouth, especially on the back of the tongue and between teeth.
  • Brushing twice a day, cleaning between teeth daily and cleaning your tongue fix most cases.
  • Dry mouth, gum disease, certain foods and smoking are other common causes.
  • Oral probiotics show some promise in small studies, but the evidence is still limited.
  • Bad breath that doesn't improve with good hygiene is worth a visit to your dentist.

Where bad breath comes from

Bad breath is very common. In most people with ongoing bad breath, the source is the mouth itself. Bacteria that live on the tongue, between teeth and under the gum line break down proteins from food, dead cells and saliva. This process releases volatile sulfur compounds (VSCs), gases such as hydrogen sulfide that smell like rotten eggs.

The back of the tongue is a major source. Its rough surface traps bacteria and debris, forming a whitish or yellowish coating where odor-producing bacteria thrive.

Common causes

Poor oral hygiene

When food particles stay in your mouth, bacteria feed on them. Plaque that builds up on teeth and along the gums adds to the problem. The American Dental Association lists poor oral hygiene as one of the most common causes of bad breath.

Gum disease

Persistent bad breath or a bad taste in your mouth can be a warning sign of gum disease (periodontitis). Bacteria collect in pockets between the gums and teeth, where brushing can't reach. Bleeding gums, gums pulling away from teeth and loose teeth are other signs.

Dry mouth

Saliva washes away food and bacteria. When your mouth is dry, odor builds up. That's why "morning breath" is worse: saliva flow drops during sleep. Many medications, mouth breathing, dehydration and some health conditions cause ongoing dry mouth.

Food and drink

Garlic, onions and some spices contain compounds that are absorbed into your bloodstream and released through your lungs, so brushing only masks them temporarily. Coffee and alcohol can also dry the mouth.

Tobacco

Smoking and chewing tobacco cause their own odor, dry the mouth and raise the risk of gum disease.

Less common causes

Sinus infections, postnasal drip, tonsil stones, acid reflux and some medical conditions, including diabetes, liver disease and kidney disease, can also cause unusual breath odor. These are less common, which is why a dentist usually checks your mouth first.

What fixes most bad breath

  1. Brush twice a day for two minutes with fluoride toothpaste.
  2. Clean between your teeth once a day with floss, interdental brushes or a water flosser.
  3. Clean your tongue. Brush or scrape the top of your tongue from back to front. Research on tongue cleaning is limited, but it's a simple, low-risk habit many dentists recommend.
  4. Stay hydrated and sip water through the day.
  5. Chew sugar-free gum to stimulate saliva, especially after meals. (Keep gum with xylitol away from dogs. It's toxic to them.)
  6. Clean dentures, retainers and mouthguards daily.
  7. See your dentist for checkups and professional cleanings.

Mouthwash can help temporarily. Some antibacterial mouthwashes reduce odor-causing bacteria, but a mouthwash that only adds a mint scent will just cover the smell for a short time.

Can oral probiotics help?

Limited evidence

Oral probiotics are lozenges or chewable tablets containing bacteria meant to live in the mouth, rather than in the gut. The most studied strains are Streptococcus salivarius K12 and M18, which naturally occur in healthy mouths.

The idea is that these "friendly" bacteria compete with odor-producing bacteria for space and nutrients. Some strains also produce natural antibacterial substances.

What the research shows: a 2022 systematic review and meta-analysis in Frontiers in Nutrition looked at four randomized controlled trials with 283 participants. Three of the four individual studies reported improvements in breath odor or VSC levels after 8 to 12 weeks. But when the results were pooled, there was no statistically significant difference between probiotics and the control groups. The authors rated the evidence as low quality and called for more clinical trials.

In short, oral probiotics may help some people, especially combined with good hygiene, but they're not a proven cure for bad breath. We cover the specific strains in more detail in Probiotics for Your Mouth: What K12 and M18 Actually Do.

How to tell if you have bad breath

It's hard to smell your own breath. Try licking the inside of your wrist, letting it dry for a few seconds and smelling it. Or ask a trusted friend or family member. Your dentist can also check for you.

When to see a dentist or doctor

Book a dental visit if bad breath continues despite good brushing, flossing and tongue cleaning, or if you notice bleeding gums, loose teeth or mouth pain. If your dentist rules out a cause in your mouth, they may refer you to a doctor to check for sinus, throat or other conditions.

The bottom line

Most bad breath starts with bacteria in your mouth and responds well to daily brushing, cleaning between teeth, tongue cleaning and staying hydrated. Oral probiotics are an interesting option with early, mixed evidence. Think of them as a possible add-on, never a replacement for the basics.