How Washington’s Marijuana DUI Laws Differ from Alcohol DUIs

If you’re stopped in Tacoma or elsewhere in Pierce County on suspicion of driving under the influence, one of the first questions you may ask is: Is a marijuana DUI treated the same as an alcohol DUI in Washington?

The short answer: No. Washington’s DUI law covers both alcohol and cannabis, but the standards, science, and defenses are very different.

Below, our Tacoma DUI defense attorneys explain how the law works, how marijuana DUIs are proven, and what you can do to protect yourself if charged.

For immediate help, call the Law Offices of Smith & White at (253) 203-1645.

Washington’s DUI Law: One Statute, Two Systems

Washington’s DUI statute — RCW 46.61.502 — makes it illegal to drive while under the influence of alcohol, cannabis, or any drug.
But within that one statute, there are actually two different systems prosecutors can use to prove a DUI.

1. The “Per Se” Standard

This is the numeric rule most people know.
If a driver’s chemical test shows:

0.08 percent blood-alcohol concentration (BAC) or higher, or
5 nanograms per milliliter (ng/mL) of delta-9-THC in whole blood
within two hours of driving, the law presumes impairment automatically.
That means the State doesn’t need to show bad driving or slurred speech—just that the test result crossed the legal threshold.

2. The “Affected By” Standard

Even without hitting those numbers, prosecutors can still convict if they prove your ability to drive was actually affected by alcohol, cannabis, or any drug.
This path relies on observations and expert interpretation—such as driving behavior, field-sobriety tests, and officer testimony.

For alcohol, the per-se rule usually drives the case because breath and blood testing are well established.
For marijuana, it’s different: THC affects everyone differently, and the 5 ng/mL number doesn’t always match real-world impairment.
That’s why most marijuana DUIs in Washington end up being argued under the broader “affected by” standard rather than just the per-se limit.

How the State Proves Impairment

Washington’s DUI law allows prosecutors to proceed under either the per-se numeric limit or the “affected by” behavioral clause.
How they build the case depends on what kind of substance is involved and what evidence they have.

Alcohol Testing

For alcohol, the process is straightforward.
If your BAC is 0.08 percent or higher, the law presumes you were impaired.
If you test below that, prosecutors can still rely on field-sobriety tests or officer observations, but those cases are less clear-cut.

Marijuana Testing

For marijuana, Washington law sets a per se limit of 5 ng/mL of active delta-9-THC in whole blood measured within two hours of driving.
A result above that level creates a legal presumption of impairment—but it’s not the only way the State can prove a marijuana DUI.
If the test result is low or disputed, prosecutors can still proceed under the “affected by” clause using other evidence.

THC behaves very differently from alcohol:

Levels fall quickly after use, meaning a delayed test may not reflect your level while driving.
THC is stored in body fat and releases slowly, so frequent users can sometimes show levels above 5 ng/mL without current impairment.
Even if the blood test is taken more than two hours later, Washington law allows the result to be used with expert testimony to estimate your level at the time of driving (RCW 46.61.502(4)(b)).

Because of these scientific issues, marijuana DUI cases rely heavily on expert interpretation and timing—not just numbers.

Other Evidence

If a blood test is unavailable or inconclusive, prosecutors may use:

Field-sobriety tests (walk-and-turn, one-leg stand, gaze tests)
Officer observations (odor of cannabis, red eyes, slowed responses, erratic driving)
Admissions of use or paraphernalia in the vehicle
Expert toxicologists who attempt to link THC levels to impairment

These cases often hinge on whether the evidence truly shows unsafe driving—or simply past marijuana use.

Your Rights After Arrest: Blood Draws, Warrants, and License Issues

Once arrested for DUI, Washington’s implied-consent law (RCW 46.20.308) comes into play.
But the process depends on the type of substance and test involved.

Breath Tests (Alcohol)

Administered after arrest on a calibrated device such as a Draeger Alcotest.
Refusing a post-arrest breath test triggers a one-year license revocation, and the refusal can be used as evidence in court (RCW 46.61.517).

Blood Tests (Marijuana and Drugs)

Blood draws for THC require a warrant or valid consent.
Warrantless blood draws are generally prohibited under Birchfield v. North Dakota (2016) and State v. Martines (2015).
If a lawful blood test shows 5 ng/mL or more of delta-9-THC, the Department of Licensing (DOL) can suspend your license.
Refusing a warrant-based blood draw can have separate legal consequences and does not necessarily prevent a DOL suspension once a valid test result is obtained.

License Hearings

You have only seven days from the date of notice to request a DOL hearing to contest a suspension.
Miss that deadline, and your suspension begins automatically 30 days after arrest.

Penalties and Consequences

Once the State proves impairment—either by per-se test result or by showing you were “affected by” a substance—marijuana DUIs carry the same penalties as alcohol DUIs under RCW 46.61.5055:

Fines, possible jail time, and probation
Driver’s-license suspension
Drug or alcohol assessment and treatment
Ignition-interlock-device (IID) requirement after conviction

There’s no “high-THC” enhancement like the “high-BAC” tier for alcohol because the statute’s penalty grid is keyed to BAC ≥ 0.15 or test refusal, not THC levels.
Judges can still impose tougher sentences when aggravating factors exist, such as prior DUIs, injury, or combined alcohol-and-cannabis use.

Special Situations: Under-21 Drivers and Medical Use

Under-21 Drivers

Washington has a separate “minor DUI” law (RCW 46.61.503) for drivers under 21.
An under-21 driver can be charged if, within two hours of driving, they have:

BAC of 0.02 or higher, or
Any detectable active THC above 0.00 but less than 5 ng/mL.

If the THC level reaches 5 ng/mL or higher, the case becomes a standard adult DUI under RCW 46.61.502.
Minor-DUI convictions are misdemeanors, carrying lighter penalties than adult DUIs but still resulting in license suspensions and a criminal record visible on background checks.

Medical Marijuana Users

Having a medical authorization does not provide a defense.
Under RCW 46.61.502(2), being legally entitled to use cannabis “is not a defense” to any DUI charge.
All drivers—medical or recreational—are held to the same 5 ng/mL standard.

Example: A Marijuana DUI in Pierce County

Imagine a Tacoma driver stopped for drifting between lanes.
The officer smells marijuana, notes delayed responses, and obtains a warrant for a blood test.
The sample—taken 90 minutes after driving—shows 6.0 ng/mL THC.

That reading exceeds the per-se limit, giving prosecutors a presumption of impairment.
But a defense attorney could challenge:

Whether the test was collected and stored properly,
Whether the sample degraded during transport, or
Whether the level at the time of driving was truly above 5 ng/mL.

If the reading had been lower—say 3 ng/mL—the State would need to rely more heavily on officer observations and expert opinion, since the “affected by” clause, not the per-se rule, would control.

Common Questions

Can I refuse a blood draw?
You can, but officers may seek a warrant. Refusing a valid warrant may have separate legal consequences.

What if the test was taken too late?
It can still be used in court. The State may use expert testimony to estimate what your level was within two hours or to prove impairment under the “affected by” standard.

Will I need an ignition interlock device?
Yes. IID requirements apply to all DUI convictions, including marijuana or drug-related ones.

How long do I have to fight my license suspension?
Seven days from the date of notice. After that, the DOL suspension starts automatically.

Why Local Experience Matters — and What to Do Next

Marijuana DUI cases combine complex science with nuanced legal standards.
A local Tacoma defense attorney familiar with Pierce County procedures, warrant practices, and toxicology evidence can identify weaknesses in the State’s case that others might miss.

If you’ve been charged with a marijuana DUI in Tacoma or anywhere in Pierce County:

Request your DOL hearing immediately (within seven days).
Do not discuss your case with police without an attorney.
Call a Tacoma DUI defense lawyer experienced with cannabis-related DUIs.
Document your timeline—when you last used cannabis, when you were stopped, and when blood was drawn.
Let your attorney review all testing and warrant paperwork for errors.

Bottom Line

Washington treats alcohol and marijuana DUIs under the same statute, but the path to proving impairment is very different.
Alcohol DUIs mostly rely on per-se numbers, while marijuana DUIs depend heavily on expert interpretation, timing, and the “affected by” standard.

If you’ve been arrested for a marijuana DUI in Tacoma or Pierce County, contact the Law Offices of Smith & White at (253) 203-1645.
Our team understands both the law and the science—and we’ll work to protect your license, your record, and your future.