"Mouth alcohol" refers to the existence of any alcohol in the mouth or esophagus. If this is present during a breath test, then the results will be falsely high. This is because the breath machine assumes that the breath is from the lungs; for complex physiological reasons, its internal computer multiplies the amount of alcohol by 2100. Thus, even a tiny amount of alcohol breathed directly into the machine from the mouth or throat rather than from the lungs can have a significant impact.

Mouth alcohol can be caused in many ways. Belching, burping, hiccupping or vomiting within 20 minutes before taking the test can bring vapor from alcoholic beverages still in the stomach up into the mouth and throat. Taking a breath freshener can send a machine's reading way up (such products as Binaca and Listerine have alcohol in them); cough syrups and other products also contain alcohol. Dental bridges and dental caps can trap alcohol. Blood in the mouth from an injury is yet another source of inaccurate breath test results: breathed into the mouthpiece, any alcohol in the blood will be multiplied 2100 times. A chronic "reflux" condition from gastric distress or a hiatal hernia can cause elevated BAC readings.