Products such as mouthwash or breath spray can ‘fool’ some breathalysers by significantly raising test results. Listerine mouthwash, for example, contains 27% alcohol.
A breathalyser is calibrated with the assumption that the alcohol is coming from alcohol in the blood diffusing into the lung, rather than directly from the mouth, so it applies a partition ratio of 2100:1 in computing blood alcohol concentration – resulting in a false high test reading.
To counter this, police officers are not supposed to administer a breath test for 15 minutes after the subject eats, vomits, or puts anything in their mouth. In addition, most instruments require that the subject be tested twice, at least two minutes apart. Mouthwash or other mouth alcohol will have somewhat dissipated after two minutes and cause the second reading to disagree with the first, requiring a retest.