Key takeaways
- K12 and M18 are strains of Streptococcus salivarius, a bacterium that naturally lives in healthy mouths.
- Both strains produce natural antibacterial proteins that can slow the growth of some odor- and cavity-related bacteria in the lab.
- Small human studies suggest K12 may reduce bad breath and M18 may help with dental plaque, but larger, independent trials are still needed.
- They're generally considered safe for healthy people. They don't replace brushing, flossing or dental care.
What oral probiotics are
Most probiotics you've heard of, such as Lactobacillus and Bifidobacterium, are meant for the gut. Oral probiotics are different. They're designed to settle in the mouth and throat, so they usually come as lozenges, chewable tablets or powders that dissolve slowly in the mouth instead of capsules you swallow.
Streptococcus salivarius is one of the first bacteria to colonize the mouth in infancy and is common on the tongue of healthy people. It's a different species from Streptococcus pyogenes (which causes strep throat) and Streptococcus mutans (linked to cavities).
Streptococcus salivarius K12
K12 was first isolated from the mouth of a healthy child in New Zealand by researchers at the University of Otago. It's sold under the ingredient name BLIS K12.
How it's supposed to work: K12 produces bacteriocins, natural antibacterial proteins called salivaricins. In lab studies, these can inhibit some bacteria linked to bad breath and throat infections. The idea is that K12 settles on the tongue and crowds out less friendly bacteria.
What human studies show:
- Bad breath: in early trials by the team that developed K12, published in Oral Diseases in 2005, people with confirmed bad breath used an antibacterial rinse followed by K12 lozenges. Eight of 13 kept lower levels of sulfur compounds on their breath for at least two weeks. These were very small, preliminary studies run by the strain's developers.
- Later trials have been mixed. A 2022 meta-analysis of four randomized trials on probiotics for halitosis, two of which used K12 and one M18, found improvements in individual studies but no significant overall difference when the results were combined. The authors rated the evidence as low quality.
- Throat and ear infections: K12 has also been studied for recurring sore throats and ear infections in children, with some promising but early results. This is outside the scope of what a supplement can claim, and children with recurring infections need a doctor's care.
Evidence: Limited
Streptococcus salivarius M18
M18 was developed by the same research group. It's sometimes sold as BLIS M18 and is often marketed for teeth and gums rather than breath.
How it's supposed to work: M18 also produces salivaricins, and it makes enzymes (dextranase and urease) that may help break down plaque and reduce the acidity that leads to cavities.
What human studies show: small trials, including some in children and in people with braces, have reported less plaque or lower levels of cavity-associated bacteria with M18. One 12-week trial in orthodontic patients, included in the 2022 meta-analysis, found a significant drop in breath sulfur compounds in the M18 group. These are encouraging signals, but the studies are small and often short.
Evidence: Limited
K12 vs M18 at a glance
| K12 | M18 | |
|---|---|---|
| Mainly marketed for | Fresh breath, throat health | Teeth and gums |
| Where it settles | Mainly the tongue | Teeth and gum surfaces |
| Key feature | Antibacterial salivaricins | Salivaricins plus plaque-related enzymes |
| Evidence strength | Limited | Limited |
Many oral health supplements combine both strains, sometimes with other probiotics or ingredients like vitamins and plant extracts.
How to read an oral probiotic label
- Look for the full strain name. "Streptococcus salivarius K12" is specific. "S. salivarius" alone isn't, and other strains don't have the same research.
- Check the CFU count (colony-forming units) per dose and whether it's guaranteed until the expiry date or only "at time of manufacture."
- Check the format. A lozenge you let dissolve slowly gives the bacteria time in your mouth. Capsules you swallow mostly bypass it.
- Watch for proprietary blends that hide the dose of each ingredient.
- Be skeptical of big promises. Claims that a product will "regrow gums," "reverse gum disease" or "end all dental problems" go far beyond the evidence.
How to use them, if you try them
Most studies have participants take the lozenge after brushing at night, letting it dissolve slowly without chewing, and not eating or drinking for about 30 minutes afterward. Some protocols start with an antibacterial mouthwash to reduce existing bacteria first. Give any product at least four to eight weeks before judging whether it works for you.
Safety
Oral probiotics using S. salivarius are generally well tolerated in healthy people in studies. As with any probiotic, people with weakened immune systems, serious illness or heart valve problems should talk to a doctor first. If you have bleeding gums, loose teeth or mouth pain, see a dentist. These can be signs of gum disease, which needs professional treatment.
The bottom line
K12 and M18 are the best-researched oral probiotics, and the science behind them is plausible. Early studies are encouraging, but the evidence is still limited and partly from small trials. If you want to try one, choose a product with clearly named strains and realistic claims, and keep up your brushing, flossing and dental visits. For the full picture on breath odor, read What Causes Bad Breath, and Can Oral Probiotics Help?