I’m under the legal limit. Can I still get a marijuana DUI?

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Many Washington drivers believe that if their blood test shows less than 5 nanograms per milliliter (ng/mL) of THC, they’re automatically safe from a marijuana DUI. Unfortunately, that assumption is wrong.

Washington’s marijuana DUI law sets 5 ng/mL as a presumptive threshold for impairment—not a guaranteed shield from prosecution. You can still be charged and convicted even when you test below that number if prosecutors can prove you were driving while affected by marijuana.

The Two Paths to a Marijuana DUI Conviction

Under RCW 46.61.502, Washington recognizes two legal theories of impairment:

1. Per Se Violation
   If your blood contains 5 ng/mL or more of active THC within two hours of driving, the law allows the jury to presume you were impaired. This is similar to the 0.08% blood alcohol concentration rule for alcohol DUIs. Prosecutors don’t need additional evidence of bad driving or failed field tests—your THC level alone can justify a conviction.
2. “Affected By” Violation
   Even below 5 ng/mL, you can be convicted if the prosecution shows that marijuana actually affected your ability to drive safely. This broader clause doesn’t depend on a number; it depends on behavior, coordination, and officer observation.

The second prong—“while the person is under the influence of or affected by intoxicating liquor or any drug”—is what allows convictions below the numeric threshold.

What Evidence Can Be Used Below 5 ng/mL?

If your THC level is under the legal limit, the State must rely on circumstantial and behavioral evidence rather than the lab report alone. Common examples include:

• Driving pattern: weaving, slow reaction times, delayed braking, or other unsafe maneuvers
• Physical signs: red eyes, odor of cannabis, impaired balance, or slow responses
• Field sobriety test results: failure to follow instructions, poor coordination, or divided-attention errors
• Officer or Drug Recognition Expert (DRE) testimony: evaluations of eye movement, pulse rate, and pupil size
• Statements or admissions: telling an officer when or how much you smoked, or possessing paraphernalia in plain sight

All of this evidence helps prosecutors argue that you were functionally impaired—even if you tested below 5 ng/mL.

Why THC Levels Don’t Tell the Whole Story

Unlike alcohol, THC doesn’t correlate neatly with impairment. Its psychoactive effects vary widely based on tolerance, body fat, metabolism, and how it’s consumed. A new user may show clear impairment at 3 ng/mL, while a daily user may test at 8 ng/mL and show no outward signs of being high.

THC also behaves differently in the body:

• It’s fat-soluble, meaning it lingers in tissue and releases slowly over time.
• Peak blood levels drop rapidly after consumption, even though subjective effects may persist.
• Frequent users can test positive for active THC long after they last used.

For these reasons, toxicology results are only one part of a marijuana DUI case. Officer observations and driving behavior often carry equal or greater weight in court.

Underage Drivers: Zero Tolerance Still Applies

If you are under 21, Washington’s “zero tolerance” law (RCW 46.61.503) means any detectable THC can lead to a Minor DUI. You don’t get the 5 ng/mL cushion. Even trace amounts—whether from a single hit or secondhand exposure—can trigger criminal charges and result in license suspension, fines, and a permanent mark on your driving record.

How Police Build a Below-Limit DUI Case

When a driver tests below 5 ng/mL, officers and prosecutors tend to emphasize qualitative evidence over quantitative numbers. For example:

1. Field Observations – The officer may testify that your speech was slow, your eyes were glassy, or your movements were uncoordinated.
2. Driving Pattern – Dashcam or witness footage showing erratic or delayed driving responses can support an impairment claim.
3. Blood Test Timing – If the test occurred hours after arrest, the prosecution may argue your THC level was higher while driving.
4. Expert Testimony – A Drug Recognition Expert may testify that your physiological responses were consistent with marijuana impairment.

These cases are more complex than per se DUIs because they depend heavily on interpretation and subjective observations—factors a defense attorney can challenge.

Defending Against a Marijuana DUI Below 5 ng/mL

A skilled DUI defense lawyer can scrutinize every step of the investigation:

• Toxicology reliability: Were your blood samples properly stored, transported, and analyzed?
• Timing of the test: Did THC levels drop significantly between the time of driving and the draw?
• Officer training: Did the arresting officer follow Drug Recognition Expert protocol?
• Field test conditions: Were you tired, injured, or on medication that affected coordination?

These questions can create reasonable doubt about whether you were truly impaired—a crucial distinction in a below-limit case.

The Bottom Line

Washington’s marijuana DUI laws don’t guarantee safety just because your THC level is under 5 ng/mL. The number is a starting point, not a legal shield. If your driving behavior, field sobriety results, or other evidence suggests impairment, prosecutors can—and often do—pursue DUI charges below the statutory limit.

For drivers under 21, any amount of THC can lead to a DUI. For everyone else, the safest approach is simple: if you’ve consumed marijuana, don’t drive until you’re confident the effects have fully worn off.

If you’re already facing a marijuana DUI charge, the attorneys at The Law Offices of Barbara A. Bowden can help you understand your test results, analyze police procedures, and fight to protect your record and your license.

 

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Sources

• RCW 46.61.502 – Driving Under the Influence
• RCW 46.61.503 – Driver Under Twenty-One Consuming Alcohol or Marijuana
• Washington State Patrol – Drug Recognition Expert Program
• Washington State Liquor and Cannabis Board – Driving and Traveling with Marijuana