Marijuana DUI in Arizona
Why THC Alone Is NotEnough for a Conviction
Marijuana is legal in Arizona.
Driving while impaired is not.
Those two statements can exist at the same time—and that’s where many people get confused, scared, and overwhelmed after a marijuana DUI arrest.
If you or someone you care about is facing a marijuana DUI in Tucson or Pima County, one of the most important things to understand is this:
The presence of marijuana in your system does not automatically mean you were impaired.
In fact, under Arizona law, impairment is the key issue—and it is often the most hotly contested part of a marijuana DUI case.
As a Tucson defense attorney who regularly handles DUI and drug-related cases, I spend a significant amount of time challenging how the State tries to prove impairment, particularly when their case relies on blood testing and forensic toxicology.
This article explains how marijuana DUI cases actually work, why toxicology evidence is far more complicated than it sounds, and how a skilled defense can make all the difference.
Marijuana DUI vs. Alcohol DUI: Why the Rules Are Different
Most people are familiar with alcohol DUIs. If your blood alcohol concentration (BAC) is 0.08 or higher, the law presumes impairment.
Marijuana DUIs do not work that way.
There is no legal THC limit in Arizona that automatically equals impairment. Unlike alcohol, marijuana affects people differently based on:
Frequency of use
Tolerance
Method of consumption (smoking vs. edibles)
Time since use
Individual physiology
Because of this variability, Arizona law requires the State to prove something more than “THC was present.”
They must prove actual impairment.
What the State Has to Prove in a Marijuana DUI Case
To convict someone of a marijuana DUI, the prosecution must show that:
Marijuana or its active component (THC) was present in the body, and
The marijuana caused impairment to the slightest degree at the time of driving
That second element—impairment—is where many cases fall apart.
Blood Testing and Marijuana: What a Toxicologist Can (and Cannot) Say
In many marijuana DUI cases, the State relies heavily on a forensic toxicologist who tested the defendant’s blood.
This can sound intimidating, but toxicology evidence has significant limitations—especially with marijuana.
THC vs. Metabolites
Blood tests typically detect:
Active THC (delta-9 THC) – the psychoactive component
THC metabolites – byproducts that remain long after impairment has ended
Here’s the problem:
THC and its metabolites can remain in the bloodstream for days—or even weeks—after use.
That means a blood test can be positive even when the person is completely sober.
There Is No Reliable THC Number That Equals Impairment
This is one of the most important points for potential clients to understand.
Unlike alcohol, there is no scientific consensus that a specific THC level equals impairment.
Even forensic toxicologists will often admit:
Two people with the same THC level can have very different levels of impairment
Chronic users may have detectable THC with no impairment
THC levels do not rise and fall in a predictable way
This makes marijuana DUI cases far more subjective—and far more defensible.
Field Sobriety Tests and Marijuana Impairment
Police often rely on field sobriety tests to support impairment allegations. However, these tests were originally developed to detect alcohol impairment, not marijuana impairment.
Many factors can affect performance on these tests, including:
Anxiety
Fatigue
Medical conditions
Injuries
Poor instructions or uneven surfaces
Field sobriety tests alone do not prove marijuana impairment, especially when toxicology evidence is weak or inconclusive.
Drug Recognition Experts (DREs): Not as Scientific as They Sound
Some marijuana DUI cases involve a Drug Recognition Expert (DRE).
While DREs receive specialized training, their evaluations are still largely observational and subjective. They are not toxicologists, and they do not determine THC levels.
A DRE’s opinion is only one piece of evidence—and it can be challenged, particularly when it is not supported by objective scientific data.
Why Forensic Toxicology Testimony Is So Important
When I interview or cross-examine a forensic toxicologist in a marijuana DUI case, the focus is often on:
The limitations of blood testing
The inability to pinpoint impairment timing
The lack of a reliable THC-to-impairment correlation
Scientific disagreement within the toxicology community
These are not loopholes. They are realities of the science.
And juries are entitled to hear them.
Marijuana DUI Cases Are Not “Open and Shut”
Many people assume that once blood results come back positive, the case is over.
That is simply not true.
Marijuana DUI cases require careful analysis of:
The traffic stop
Officer observations
Field sobriety testing
Blood draw procedures
Toxicology methods
Expert testimony
When these cases are properly examined, weaknesses often emerge.
Why Experience Matters in Marijuana DUI Defense
Defending a marijuana DUI is not the same as defending an alcohol DUI.
It requires:
Understanding forensic toxicology
Knowing how to question expert witnesses
Staying current on evolving marijuana science
Challenging assumptions rather than accepting them
Not every defense attorney is comfortable pushing back against expert testimony. I am.
If You Are Facing a Marijuana DUI in Tucson or Pima County
A marijuana DUI charge can feel overwhelming, especially when you are told that “the blood test says it all.”
It doesn’t.
Impairment must be proven, and marijuana cases are often far more defensible than people realize.
If you or a loved one has been charged with a marijuana DUI in Tucson, Pima County, or surrounding areas, getting legal advice early matters. Evidence can be challenged, experts can be questioned, and outcomes can change.
Final Thought
Marijuana DUI cases sit at the intersection of law, science, and assumption. When those assumptions are challenged, the State’s case is often weaker than it appears.
If you are looking for a defense attorney who understands both Arizona DUI law and forensic toxicology, you deserve someone who will take the time to dig into the details—not just accept the paperwork at face value.
