Listerine is an antiseptic mouthwash that kills bacteria in your mouth using a blend of four essential oils: eucalyptol, thymol, methyl salicylate, and menthol. In clinical trials, it reduces plaque buildup by 14% to 34% and gingivitis by 22% to 34% compared to a placebo. It’s one of the most widely used over-the-counter mouthwashes in the world, and the original formula carries the American Dental Association Seal of Acceptance for helping prevent and reduce gingivitis and plaque above the gumline.
How It Kills Bacteria
The four active ingredients in Listerine are plant-derived compounds, each present in small concentrations: 0.092% eucalyptol, 0.064% thymol, 0.06% methyl salicylate, and 0.042% menthol. These compounds work together to disrupt bacterial cell walls. Because the oils contain a wide variety of chemical components, bacteria have a harder time developing resistance to them compared to a single-target antibiotic. The combination matters: the oils interact with each other, and their collective effect is greater than any single ingredient alone.
The original Listerine Antiseptic also contains 26.9% alcohol (ethanol), which serves as a solvent to keep the essential oils dissolved in the liquid. Cool Mint Listerine contains 21.6% alcohol. The alcohol itself contributes some antimicrobial action, but the essential oils remain the primary active agents. Listerine also makes alcohol-free versions for people who find the burning sensation uncomfortable or who want to avoid alcohol for other reasons.
What It Does for Plaque and Gum Disease
Plaque is the sticky film of bacteria that builds up on your teeth throughout the day. Left alone, it hardens into tarite and inflames your gums, a condition called gingivitis. Listerine’s main clinical benefit is reducing both of these problems. The 14% to 34% reduction in plaque and 22% to 34% reduction in gingivitis observed in long-term studies represent meaningful improvements on top of regular brushing and flossing. It doesn’t replace either of those, but it reaches areas between teeth and along the gumline that a toothbrush can miss.
The ADA’s acceptance of Listerine Antiseptic is specifically based on evidence that it helps prevent and reduce gingivitis and plaque above the gumline when used as directed. That typically means swishing 20 milliliters for 30 seconds, twice a day.
How It Affects Your Oral Microbiome
Your mouth is home to hundreds of bacterial species, and not all of them are harmful. This is where Listerine’s effects get more complicated. A substudy of the PReGo trial found that after three months of daily Listerine Cool Mint use, the overall composition of the oral microbiome shifted significantly. Two species in particular became more abundant: Fusobacterium nucleatum and Streptococcus anginosus.
That finding raised concern among researchers because both of these bacteria have been linked to periodontal disease and, in broader medical research, to conditions like esophageal and colorectal cancer. The study authors concluded that “the regular use of Listerine mouthwash should be carefully considered.” This doesn’t mean Listerine causes these diseases. It means the mouthwash doesn’t kill all bacteria equally, and the ones that survive and thrive may not be the ones you’d want to encourage. For people using Listerine to manage a specific gum problem, the trade-off likely makes sense. For people using it casually “just in case,” this is worth knowing.
Common Side Effects
Listerine is generally well tolerated, but side effects are more common than most people realize. In one systematic review, 62% of patients using essential oil mouthwashes reported at least one adverse effect, compared to 32% in control groups. The most frequently reported issues were:
- Tooth staining and tarite buildup
- Changes in taste perception
- Altered oral sensation
- Burning or mouth ulcers
- Soft tissue irritation
Many of these effects showed up within two weeks of regular use. Interestingly, one study found that the alcohol-free essential oil formula actually caused more reported side effects (35% of users) than the alcohol-containing version (7%), suggesting that alcohol content isn’t the only factor driving irritation. Soft tissue changes in the mouth were more common after about a month of use but tended to decrease by the six-month mark, suggesting the mouth adapts over time.
Timing It With Brushing
If you brush with fluoride toothpaste and then immediately rinse with Listerine, you’re washing away the fluoride before it has time to strengthen your enamel. Fluoride needs at least 15 minutes of contact with your teeth to do its job effectively. Most mouthwashes, including many Listerine products, contain lower concentrations of fluoride than toothpaste, so rinsing right after brushing actually reduces the protection you get.
The simplest fix is to use Listerine at a different time than you brush. You could use it after lunch, for example, and brush in the morning and at night. If you prefer to use it during your regular brushing routine, wait at least 15 minutes after spitting out your toothpaste before rinsing with mouthwash.