Mouth Alcohol: A Defence, Not a Special Reason, in Drink Driving Cases

At Geoffrey Miller Solicitors, we often act in cases where clients are charged with driving with excess alcohol, but where the breath alcohol reading does not accurately reflect the amount of alcohol actually consumed. One of the most misunderstood areas of motoring law is the legal effect of mouth alcohol and how it can invalidate a prosecution. Contrary to common belief, mouth alcohol is not only a potential “special reason” for committing the offence; it can amount to a complete defence.

What Is Mouth Alcohol?

Mouth alcohol refers to residual alcohol in the upper respiratory tract, usually caused by regurgitation, belching, reflux, or recent consumption. Breath testing devices are designed to measure alcohol from deep lung air (alveolar breath). However, if alcohol is present in the mouth or throat, even in small amounts, it can create an inflated reading and undermine the validity of the test result.

The Legal Foundation for a Mouth Alcohol Defence

The key legal test under section 5(1)(a) of the Road Traffic Act 1988 is whether a driver consumed “so much alcohol that” the proportion in their breath exceeded the prescribed limit. This requires a direct and reliable relationship between the alcohol consumed and the measurement produced.

In Cracknell v Willis [1988] RTR 1, the House of Lords held that a defendant may challenge the reliability of a breath test result by showing that the machine produced a false reading, even if no direct evidence of mechanical malfunction exists. The decision made clear that there is no statutory presumption of infallibility for breath testing instruments. If evidence suggests the reading was distorted by mouth alcohol, it may be excluded or disregarded entirely.

Similarly, in Stephen Decani v City of London Magistrates’ Court [2017] EWHC 3422 (Admin), an appeal successfully pursued by Geoffrey Miller Solicitors, the High Court confirmed that defence arguments about mouth alcohol or inaccurate machine readings are valid issues of law and fact, particularly where the suspect suffers from digestive issues or has recently vomited or regurgitated.

Not Limited to a Special Reason – A Full Defence

Some assume that mouth alcohol can only be raised in mitigation or as a “special reason” to avoid disqualification. That is not correct. Where the breath test result is shown to be unreliable or misleading, and where the actual alcohol consumed would not have caused the reading to exceed the legal limit, the defendant is not guilty of the offence.

This distinction is crucial. A special reason is an explanation that may avoid the mandatory minimum penalty (a driving ban) even where the offence is admitted. A defence, by contrast, results in an acquittal and no penalty.

The defence of unreliable breath readings due to mouth alcohol is therefore one of factual innocence. If the court accepts that alcohol in the mouth or throat caused an inflated reading, and that the person had not actually consumed enough alcohol to exceed the limit, then the prosecution will fail to prove an essential element of the offence.

Section 78 PACE 1984 – Excluding Unfair Evidence

In appropriate cases, the defence may also apply to exclude the breath test evidence under Section 78 of the Police and Criminal Evidence Act 1984. This provision allows the court to exclude prosecution evidence where its admission would have such an adverse effect on the fairness of proceedings that it ought not to be admitted.

If, for example, the evidential procedure was carried out improperly, the statutory warnings were not effectively given, or the instrument was not operated in line with protocol, the defence may argue that the breath reading is unreliable and should be excluded.

Courts have accepted this principle in multiple cases. In particular, Cracknell v Willis confirms that if the reliability of the breath test is genuinely in question, it would be unfair to allow the court to rely on it. The principle was further upheld in Decani, where expert evidence supported a challenge to the validity of the reading.

WARNING

Misleading Sales Tactics Around the MGDD/A Form

Some legal representatives or firms use the MGDD/A procedure (Manual of Guidance on Drink and Drug Driving, version A) to sell the idea that breath tests are routinely flawed due to officers not reciting or reading a so-called “25-page booklet” in full. This is a misleading sales tactic.

It is true that the MGDD/A form must be properly completed and that errors can, in some cases, lead to a challenge in court. However, it is simply not the case that the entire document is read aloud to suspects or that a failure to read out every question verbatim amounts to a defence. In fact, the MGDD/A booklet is structured so that if a suspect answers “Yes” or “No” to certain questions, it will skip several other questions, which is why a suspect may only experience a fraction of the form being read to them.

The form does not operate as a literal script. Its purpose is to ensure legal compliance, not to serve as a 25-page monologue. Suggesting otherwise during an initial sales call is designed to create the impression that the firm offering the service has identified a technicality that can guarantee an acquittal. In reality, the law is never that simplistic. Successful defences are built on evidence, analysis, and experience, not superficial oversights.

If you are curious, you can view the MGDD/A form used in England and Wales here:

MGDD/A Breath Procedure Form (official link)

We routinely review the proper use of the MGDD/A form when preparing a defence. However, we do so with a view to substance over sales tactics.

The Burden of Proof and Procedural Safeguards

It is the prosecution who must prove that the test was carried out correctly and that the result is reliable. The defence may challenge the presumption of proper functioning by raising reasonable doubt – for example, by adducing evidence of:

1. Digestive conditions such as reflux.
2. Failure to observe the correct pre-test procedure, such as insufficient observation time.
3. Irregularities in custody – for example, failures to provide appropriate medical care or to communicate statutory rights.
4. Contaminating factors such as post-incident alcohol consumption or failure to delay testing after regurgitation.

We Are Experts in Challenging Unreliable Readings

At Geoffrey Miller Solicitors, we have successfully defended clients across the UK where the prosecution relied on elevated breath test readings later shown to be unreliable. In many of these cases, the police failed to follow proper procedures, or the individual had a medical or physiological condition that affected the result.

We do not rely on generic arguments or templates. Our legal team scrutinises every stage of the case,  from the roadside stop to the station procedure, to identify technical and evidential flaws that can make all the difference.

If you have been charged with drink driving but believe your breath test may not reflect what you actually drank, please get in touch.