Challenge the Evidence
DUI Defense Strategies
A DUI charge is not a conviction. The prosecution must prove every element beyond a reasonable doubt — and each element, from the stop itself to the reliability of the testing, can be challenged. These are the defenses that come up most often.
Every defense below starts the same way: with the records. Police reports, maintenance logs, lab packets, and body-camera footage are all discoverable. You — or your attorney — can request them, and the Motion Bank includes templates for doing exactly that.
Breath Test Challenges
Calibration & Maintenance Issues
Breath-testing devices must be calibrated and maintained on strict schedules. You can request the maintenance records and calibration data to show whether the device was functioning properly when it measured your BAC.
Operator Error
The operator must follow specific procedures and hold a valid certification. Training deficiencies, improper testing protocol, or mishandling of the device can render the results unreliable.
Medical Conditions: GERD & Diabetes
Gastroesophageal reflux disease (GERD) and diabetes can cause falsely high readings. These conditions put residual alcohol or ketones in the mouth or breath that do not reflect blood alcohol content, artificially inflating the result.
Mouth Alcohol
Recent drinking, mouthwash, breath spray, or other sources of mouth alcohol can produce artificially elevated readings. In those moments the device is reading mouth alcohol, not blood alcohol.
Rising BAC Defense
Your BAC may still have been rising at the time of the test. If you drank shortly before driving, your BAC could have been below the legal limit while driving but above it by the time you were tested.
Blood Draw Challenges
Blood cases turn on procedure. Schmerber, McNeely, and Birchfield set the constitutional rules; Arizona adds a statutory right to an independent test under A.R.S. § 28-1388(C).
Chain of Custody Violations
Blood samples must be properly handled, labeled, and stored. Any break in the chain of custody can compromise the sample's integrity and give the defense grounds to attack — or move to suppress — the result.
Improper Collection Procedure
Blood draws must be performed by qualified personnel using proper sterile technique. Improper site preparation, contamination, or problems with the gray-top tube's preservative and anticoagulant can affect accuracy.
Laboratory Errors
Lab equipment must be maintained and calibrated, and headspace gas chromatography has known sources of measurement uncertainty. You can request the lab's protocols, QA/QC records, and the analyst's bench notes in discovery.
Timing Issues
The time between driving and the blood draw matters. Delays allow alcohol to be absorbed or metabolized, and the state's retrograde extrapolation back to the time of driving can be challenged by the defense.
Field Sobriety Test Challenges
Non-Standardized Testing Conditions
Field sobriety tests are validated only under controlled, standardized conditions. Uneven pavement, poor lighting, passing traffic, cold weather, or bad footwear can affect performance and undermine the results.
Officer Training & Certification
Officers must administer the three NHTSA-standardized tests — HGN, walk-and-turn, and one-leg stand — exactly as trained. Improper instructions or deviation from the standardized procedures undermines reliability; even under lab conditions NHTSA's own studies put accuracy at roughly 77%, 68%, and 65% respectively.
Medical & Physical Conditions
Inner-ear disorders, arthritis, back or leg injuries, obesity, neurological conditions, medications, and age can all degrade performance on field sobriety tests for reasons that have nothing to do with alcohol.
Constitutional Challenges
Illegal Traffic Stop
An officer must have reasonable suspicion of a violation before initiating a traffic stop. If the stop lacked a legal basis, all evidence obtained afterward may be suppressed under the Fourth Amendment.
Lack of Reasonable Suspicion to Expand the Stop
Even after a lawful traffic stop, an officer needs independent reasonable suspicion of impairment before prolonging the detention for a DUI investigation. An ordinary traffic violation alone does not justify it.
Miranda Violations
If you were in custody and questioned about your driving or drinking without being advised of your Miranda rights, statements obtained in violation of those rights may be excluded from evidence.
Prescription Drug Defense
A.R.S. § 28-1381(D) provides a statutory defense: a person cannot be convicted of drug DUI for a substance prescribed by a licensed medical practitioner if it was used as directed. The defense requires documentation of the prescription, proof the directions were followed, and evidence the drug was prescribed to you specifically. Medical-expert testimony often supports it.
Post-Proposition 207 Marijuana Defense
After Proposition 207, Arizona’s recreational marijuana law, a marijuana DUI requires proof of actual impairment. Unlike alcohol, the mere presence of marijuana does not establish impairment.
Prosecutors must show that marijuana actually affected your driving. THC can remain detectable long after impairment has worn off, which makes a positive test alone a weak indicator of impairment at the time of driving.
The defense can attack the impairment evidence through cross-examination of the officers, expert testimony, and analysis of the driving conduct and field-test performance.
Every Case Is Defensible
The prosecution must prove every element beyond a reasonable doubt. Investigating the entire case — from the initial stop through testing and lab analysis — is how weaknesses get found. Early action matters: evidence gets preserved, witnesses get located, and in Arizona you have only 30 days to request an MVD hearing on your license suspension (A.R.S. § 28-1385). See penalties by charge tier and what to do in the first 24 hours.
Put These Defenses to Work
The Motion Bank includes court-ready templates for suppression motions, discovery requests, and evidentiary challenges covering every strategy on this page. A free account gets you started.