A breath machine is supposed to measure alcohol deep in your lungs. When alcohol is lingering in your mouth instead, the machine reads it as if it came from your blood, and the number can spike far above your true BAC. That is the mouth alcohol defense. I am Joel Brand, and it is one of the most common reasons a breath result is simply wrong.
Why mouth alcohol throws off the result
An evidentiary breath machine is designed to capture "deep lung" or alveolar air, the air from the bottom of your lungs that reflects the alcohol actually circulating in your blood. It assumes the sample is uncontaminated. But if even a tiny amount of alcohol is sitting in your mouth, throat, or esophagus when you blow, that concentrated residue mixes into the sample and the machine cannot tell the difference. Because mouth alcohol is so much more concentrated than the alcohol in deep lung air, a trace amount can drive the reading dramatically higher than your real blood alcohol level, turning a sub-limit driver into an apparent "fail."
What causes mouth alcohol
Burping, belching, hiccupping, and acid reflux or GERD can all bring alcohol up from the stomach into the mouth. So can dental work, dentures, braces and orthodontics that trap fluid, recent use of mouthwash or breath spray, chewing tobacco, and a drink taken shortly before the test. None of it reflects what is actually in your bloodstream. A person who took their last sip minutes before being stopped, or who burped on the way to the station, can carry mouth alcohol straight into the breath test.
Acid reflux and GERD are common culprits
Gastroesophageal reflux disease deserves special mention because it is so widespread and so often overlooked. In someone with GERD or a hiatal hernia, stomach contents, including any alcohol, rise back up into the esophagus and mouth involuntarily, sometimes without the person even noticing. If that happens during or just before the breath test, it injects mouth alcohol into the sample. Many of my clients have no idea their reflux had anything to do with their reading until we connect the medical history to the science. See other medical conditions that affect a DUI.
The 15-minute observation rule
To guard against exactly this, the officer is required to continuously observe you for about 15 minutes before an evidentiary breath test, making sure you do not burp, regurgitate, eat, drink, or smoke. The point of the rule is to let any mouth alcohol dissipate before the sample is taken. In the real world that observation is often cut short or not done properly. The officer is filling out paperwork, walking you to the machine, or simply not watching closely, and a silent burp or reflux event goes unnoticed. When the observation period was not genuinely honored, the result is open to challenge.
Two samples that disagree are a red flag
Title 17 requires two separate breath samples that must agree within a set tolerance. Mouth alcohol often reveals itself precisely here: the contaminated sample reads high and the cleaner one reads lower, producing a discrepancy that should not happen with a properly administered test. A meaningful gap between the two readings is a signal that something contaminated the sample, and it is one of the first things I look for in the breath records.
Mouthwash, sprays, and other hidden sources
Many everyday products contain alcohol and can sabotage a breath test. Common mouthwashes can be 20 to 25 percent alcohol, and a swish shortly before a test can leave residue that reads alarmingly high. Breath sprays, cough drops, cold and flu medicines, and even some chewing gums contain alcohol. People also forget that certain oral conditions and appliances trap fluid: dentures, bridges, crowns, and orthodontic hardware can all hold a reservoir of alcohol-laden saliva long after the last drink. An officer who does not ask about any of this, and a 15-minute observation that did not actually rule it out, leaves the door wide open to this defense.
How long mouth alcohol lasts
The reason the observation period exists is that mouth alcohol does dissipate, but it takes time. Research generally shows that mouth alcohol can affect a breath reading for roughly 15 to 20 minutes after it is introduced, which is exactly why the rule calls for continuous observation over that window. If the test was administered too soon after a drink, a burp, a reflux episode, or the use of an alcohol-containing product, and the observation was not truly continuous, the residual alcohol had not cleared. That timing problem is frequently the difference between a number that convicts and a number that should never have been trusted.
How I use it
I look at whether the observation period was actually honored, your medical and dental history (reflux, GERD, dentures, and recent dental work all matter here), the timeline of your last drink, and the breath records themselves, including the two-sample agreement. Where the science supports it, I push for the blood result instead, or argue that the breath number is too unreliable to support the charge. It pairs naturally with the calibration defense and the rising BAC defense.
Why it matters in court and at the DMV
This defense is just as live at the DMV hearing as in the criminal case, because the DMV also relies on the breath number to justify the suspension. It applies directly to a VC 23152(b) charge, where the breath result is the entire case. If that number is unreliable, the case built on it is too, and a breath result inflated by mouth alcohol is exactly the kind of unreliable evidence that should not decide whether you are convicted. See my top DUI defenses.
Think this applies to you? Let's find out.
Whether this defense fits depends on the specific records and facts of your stop, which is exactly what I review. Use the free case analysis on this page, or call me directly at (888) 271-6644. I answer my own phone, 24/7.
From the DUI blog: Could Mouthwash or Breath Spray Trigger a False DUI Reading in California?.