Yes, mouthwash can affect a breathalyzer test, and the effect is larger than most people expect. In a controlled study, subjects who rinsed with Listerine (26.9% alcohol) blew an average breath-alcohol reading of 0.24 just two minutes later — three times the 0.08 legal limit used in all 50 states.1PubMed. Breath Alcohol Values Following Mouthwash Use The readings fell below the legal threshold within about ten minutes, but for that short window a person who had consumed no alcohol at all could fail a roadside test.
Why Mouthwash Fools the Device
A breathalyzer is built to measure alcohol coming out of your lungs. As blood circulates through the lungs, some alcohol evaporates into the air sacs and gets exhaled; the device measures that exhaled alcohol and converts it to a blood alcohol estimate using a standard ratio of 2,100 parts blood to 1 part breath. The math works when the only alcohol in your breath came from your bloodstream.
Mouthwash breaks that assumption. Swishing an alcohol-based rinse leaves a thin film of ethanol on your tongue, gums, and cheeks. The breathalyzer cannot separate this residual “mouth alcohol” from alcohol that traveled through your bloodstream into your lungs. It reads all the alcohol in your exhaled air the same way and reports a number, so the result reflects what is sitting in your mouth rather than what is in your blood.
The alcohol content in commercial mouthwash is not trivial. Listerine Antiseptic contains 26.9% alcohol by volume, and Cool Mint Listerine sits at 21.6%. Most beers run 4–6% and most wines 12–15%, so a strong antiseptic rinse is closer in concentration to hard liquor than to beer, even though you spit it out.
How High the Readings Go and How Fast They Fade
A study published in the Journal of Analytical Toxicology measured breath-alcohol readings at timed intervals after subjects rinsed with three common mouthwash brands. Two minutes after rinsing:
- Listerine (26.9% alcohol) produced an average reading of 0.24 — three times the legal limit.
- Scope (18.9% alcohol) produced an average reading of 0.17 — more than double the legal limit.
- Lavoris (6.0% alcohol) produced an average reading of 0.036 — below the legal limit but still detectable in someone who had consumed no alcohol.
All three brands dropped below 0.08 within ten minutes, and the decay followed an exponential curve, falling fast at first and then tapering.1PubMed. Breath Alcohol Values Following Mouthwash Use The practical danger zone is roughly the first ten to fifteen minutes after rinsing.
The Observation Period Is Supposed to Prevent This
Most states require the officer administering an evidentiary breath test to continuously observe the person for 15 to 20 minutes beforehand. During that window, the person cannot eat, drink, smoke, vomit, or put anything in their mouth. The logic is simple. If no new alcohol enters the mouth, residual mouth alcohol should dissipate before the test begins, which matches what the research shows about how quickly mouthwash readings fall.
When the observation period is followed properly, it eliminates most mouthwash interference. The problem is that “continuous observation” is not always continuous in practice. An officer who steps away, fills out paperwork with their back turned, or starts the clock before the person finishes burping has potentially compromised the test. A failure to follow the observation protocol can become grounds to challenge a breath test result in court.
The Machine’s Own Safeguard Is Not Foolproof
Modern evidentiary breathalyzers use software algorithms called slope detectors that are supposed to catch mouth alcohol. As you blow, the device monitors how the alcohol concentration in your breath changes across the exhale. Normal deep-lung alcohol produces a smooth curve that rises and levels off. Mouth alcohol tends to produce spikes, dips, or wavy patterns that look different from a clean sample.2NCBI (National Center for Biotechnology Information). The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing: Case Reports
In practice, the track record is uneven. One evaluation found a slope detector’s ability to identify mouth alcohol was only about 52%. A 2025 study documented three cases where mouth alcohol contamination went undetected: in one, the breath profile showed a visible aberration from normal but the machine never flagged it; in another, the device only caught the contamination on the very last oscillation of the sample, after already processing earlier portions without any alert.2NCBI (National Center for Biotechnology Information). The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing: Case Reports The assumption that the machine will catch its own errors is not always correct.
Mouthwash is the most commonly cited cause of mouth alcohol, but it is not the only one. Acid reflux, ketosis from diabetes or a low-carb diet, and even some asthma inhalers have been shown to produce elevated readings in people who had not been drinking. The observation period is the same practical protection against all of them.
What to Do If You Were Pulled Over Shortly After Rinsing
Tell the officer. Disclosing recent mouthwash use is not an admission of guilt; it is relevant information that should trigger a proper observation period before any evidentiary test. Officers are trained to account for mouth alcohol, and an unexplained high reading on your record is a far worse outcome than a brief conversation at the roadside.
If at least fifteen minutes have passed since you rinsed, the risk of a false reading is minimal based on the available research. Rinsing with water can help clear residual alcohol faster, but time is the reliable variable. If a roadside screening produces a reading that seems inconsistent with how much you actually drank, the station’s evidentiary instrument, administered after a proper observation period, should produce a more accurate result.
One boundary worth knowing: refusing the test is not a workaround. Every state has an implied consent law, meaning your driver’s license already commits you to submit to chemical testing when lawfully requested. Refusal typically triggers automatic license suspension and can be used against you in court, so declining a breath test because you used mouthwash is almost certainly worse than explaining the situation and taking the test after an appropriate wait.
Alcohol-Free Mouthwash Removes the Problem
The simplest way to eliminate this risk is to use an alcohol-free mouthwash. Several major brands now sell alcohol-free formulations, including versions of Listerine, CloSYS, and TheraBreath. These products use alternative antimicrobial agents such as cetylpyridinium chloride in place of ethanol, and they will not register on a breathalyzer regardless of timing. If mouthwash is part of your daily routine and you drive regularly, switching formulas takes one more variable out of an already stressful traffic stop.