Mouth Alcohol as a DUI Defense

“Residual mouth alcohol" is a common defense to a DUI charge. The name refers to the traces of alcohol that remain in the mucosal linings of the mouth after someone consumes alcohol.

This alcohol will be detected by a DUI breath test, even though it has not yet been absorbed into the bloodstream. This can cause the DUI breath testing device to report a falsely high blood alcohol concentration (“BAC”).1

As result, an innocent person might be charged with a crime such as:

To help you better understand the “drunk driving” defense of residual mouth alcohol, our California DUI defense lawyers discuss, below:

california mouth alcohol dui legal defense attorneys breath spray
Breath sprays containing alcohol can create “residual mouth alcohol” that can create a falsely high BAC result on a California DUI breath test.

1. What is “residual mouth alcohol”?

“Mouth alcohol” consists of traces of alcohol that remain in the mucosal linings of the mouth. It is created whenever someone consumes (or regurgitates) anything with alcohol in it.

Common sources of mouth alcohol include:

  • Alcoholic beverages,
  • Medical conditions like gastroesophageal reflux disease (GERD), acid reflux and heartburn,2
  • Medicines containing alcohol (such as Nyquil),
  • Chewing tobacco,
  • Mouthwashes or breath sprays containing alcohol, and
  • Dental work, such as dentures or retainers, that trap (and release) consumed food and liquid.

2. How mouth alcohol leads to falsely elevated BAC results

When someone consumes anything containing alcohol, some of the alcohol remains in the mucosal linings of the mouth. This alcohol dissipates quickly fairly quickly—typically about ten minutes after consumption.3

But it takes 30 minutes or more on average for this alcohol to enter the bloodstream. So immediately after consumption, the alcohol is present in the driver's mouth but has not yet raised the drinker's BAC.

How mouth alcohol “tricks” a breath testing machine

Breath testing devices are meant to measure “alveolar” (deep lung) air.4 The “alveoli” are air sacs located deep within the lungs. They are surrounded by capillaries, blood vessels no more than 1/1000th of a millimeter thick.

Capillaries are so thin they allow gases to pass from the lungs into the bloodstream and vice versa. This includes alcohol vapor. So, a portion of any alcohol in the bloodstream can cross into the lungs.

capillaries human lung mouth alcohol dui defense california
Capillaries beneath the lungs allow alcohol to pass from the bloodstream into the lungs. This alcohol can then be detected by a DUI breath test.

When a person blows into a breath testing device, some of this alcohol vapor mixes with the exhaled air. The DUI breath testing machine measures this alcohol and mathematically converts the measurement into a roughly equivalent BAC.5

Unfortunately, breath testing machines cannot always distinguish “mouth air” from “alveolar” air.6

So if there is alcohol in the mouth, it will get mixed into the sample. This artificially raises the person's BAC measurement.

3. Is it possible for someone who did not drink to test positive on a DUI breath test?

Yes. Anything that contains alcohol can leave residual alcohol in the mouth. This can lead to a “positive” DUI breath test.

Products besides alcoholic beverages that often contain alcohol include cold medicines, chewing tobacco, mouthwashes and breath spray.

And in rare cases, mouth alcohol can also result from medical conditions such as GERD and acid reflux. This is most likely to occur shortly after the alcohol is consumed.7

Any of these factors can raise a driver's BAC into the “danger zone” for criminal charges.

GERD mouth alcohol dui legal defense california
GERD and acid reflux allow ingesting alcohol to “backwash,” leading to “mouth alcohol” that can artificially inflate BAC on a DUI breath test.

4. California's 15-minute observation period for a DUI breath test

California law requires law enforcement officers to observe a DUI subject for 15 continuous minutes before beginning an “evidentiary” (post-arrest) DUI breath test.8 (This requirement does not apply to a preliminary alcohol screening (“PAS”) breath test that is given on a handheld “breathalyzer.”)

This 15-minute observation period is mandated by Title 17 of the California Code of Regulations (“CCR”). 17 CCR sets forth procedures law enforcement must follow during California DUI blood tests, breath tests and urine tests.9

The rationale behind the 15-minute observation period for a DUI breath test

Mouth alcohol generally dissipates within 15 minutes, as long as no other alcohol is introduced into the mouth. So by observing a driver continuously for 15 minutes, an officer can make sure the driver does not:

  • Drink,
  • Eat,
  • Smoke,
  • Use breath spray,
  • Put anything in his/her mouth,
  • Burp,
  • Regurgitate, or
  • Vomit (which could bring alcohol from the stomach into the mouth).

But as Pasadena DUI defense attorney John Murray10 explains:

"Law officers frequently fail to honor the 15-minute observation requirement. Many of them wait for 15 minutes before starting the test, but use the time to complete paperwork or check their phones. And even diligent officers can miss a burp or regurgitation that could cause mouth alcohol."

california police questioning suspect dui mouth alcohol attorneys
California law requires officers to observe a suspected drunk driver continuously for 15 minutes before beginning a DUI breath test.

5. Using residual mouth alcohol to fight DUI charges

Experienced California drunk driving lawyers can use mouth alcohol as a DUI defense when:

  • An officer did not wait a full 15 minutes before beginning a DUI breath test,
  • The officer waited 15 minutes but did not observe the driver continuously during that time, or
  • There is evidence that there was another possible source of mouth alcohol (such as alcohol regurgitated due to GERD or other digestive problems).

6. The prosecution claims that mouth alcohol is not a valid legal defense. Is this true?

No. While mouth alcohol is not always the most applicable of California DUI defenses, it is still a valid one. Prosecutors will point to studies that show that mouth alcohol does not contaminate a properly done DUI breath case.

But DUI breath tests are often not done properly. And the burden is on the prosecution to establish guilt beyond a reasonable doubt.

An experienced DUI defense attorney and, if needed, an expert witness can often find holes in police procedure. This is often enough to get a favorable result such as:

Charged with a California DUI? Call us for help…

california dui legal defense attorneys
Call us at 855-LAWFIRM

If you or someone you care about was charged with driving under the influence, we invite you to contact us for a free consultation.

Call us at 855-LAWFIRM to discuss your defense options with an experienced California drunk driving attorney.


We also have offices in Las Vegas and Reno offices that are skilled at the application of mouth alcohol as a DUI defense in Nevada.

Legal References:

  1. See C.D. Simpson, “Effects of Mouth Alcohol on Breath Alcohol Results,” Western Michigan University, Feb 25, 2016.

  2. See, e.g., Johnson, Voas, Kelley-Baker, Furr-Holden, “The Consequences of Providing Drinkers with Blood Alcohol Concentration Information on Assessments of Alcohol Impairment and Drunk-Driving Risk,”

    (“Extremely high BACs often indicate the presence of mouth alcohol (rather than alcohol absorbed into the drinker's system)”), J Stud Alcohol Drugs. 2008 Jul; 69(4): 539–549.

  3. See, K. Sterling, “The rate of dissipation of mouth alcohol in alcohol positive subjects,” J Forensic Sci. 2012 (“The average time for subjects to reach their unbiased BrAC was 9.35 min…”).

    See also J. Wigmore, “Alcohol testing in the workplace,” The Forensic Toxicology of Alcohol and Best Practices for Alcohol Testing In The Workplace, December 2014 (“Alcohol in the oral cavity from a recent drink or use of an alcohol-containing compound can falsely elevate the breath alcohol test results as the residual alcohol in the mouth is added to the alcohol exhaled from the lungs. This effect lasts for approximately 10 minutes…).

  4. 17 California Code of Regulation (“CCR”) 1215 (r): “'Sample' or ‘Specimen' means a representative portion of blood, urine, tissue, an artificially constituted material, or a portion of expired breath which is essentially alveolar in composition obtained for the purpose of measuring its alcohol concentration.”

  5. 17 CCR 1220.4(f): “A breath alcohol concentration shall be expressed as the number of grams of alcohol per 210 liters of breath.”

  6. Wigmore, endnote 3.

  7. See Kechagias, Jönsson, Franzén, Andersson, and Jones, “Reliability of breath-alcohol analysis in individuals with gastroesophageal reflux disease,” J Forensic Sci. 1999 Jul;44(4):814-8.

  8. 17 CCR 1221.2(a)(3)(A)(iv).

  9. 17 CCR 1215 and subsequent sections.

  10. Pasadena DUI defense attorney John Murray is a leading expert in California DUI defense, including the use of mouth alcohol as a defense against DUI charges. He has extensive experience both in the court systems of Los Angeles County and Ventura County and in California DMV hearings.

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