You can use cannabis legally, feel completely sober behind the wheel, and still be charged with a DUI.

Here's why. Washington law sets a legal THC blood limit of 5 nanograms per milliliter, measured within two hours of driving. Exceed that threshold, and you can be charged regardless of whether you were actually impaired. Fall below it, and you can still be charged if an officer observes signs of impairment. For regular cannabis users, the per se limit alone is easy to exceed hours after your last use.

If this has happened to you, one thing matters most right now: a marijuana DUI in Seattle is not an automatic conviction. It can be challenged.

At Lewis and Laws, our attorneys have defended Seattle clients against DUI charges, including marijuana DUIs, where the science, the stop, and the test results are all worth challenging. If you've been charged, call us today for a free consultation.

The 5 ng/mL Limit Was Never Great Science

When Washington lawmakers needed a THC threshold for DUI, they chose 5 ng/mL. The problem? There's no solid scientific consensus that this number reliably predicts impairment.

The American Automobile Association (AAA) studied this extensively and concluded that per se THC limits (fixed cutoff numbers like 5 ng/mL) are not supported by research as a reliable indicator of driving impairment. Unlike alcohol, where the relationship between BAC and impairment is well-established, THC behaves differently in the body.

Research suggests some people are meaningfully impaired well below the 5 ng/mL mark, while daily users can function normally well above it, because tolerance changes the picture entirely. The number alone doesn't tell the story.

Why Regular Users Are Especially Vulnerable

THC is fat-soluble. It doesn't leave your system the way alcohol does. In frequent users, THC can linger in the bloodstream for days, long after any psychoactive effects have passed. Someone who uses cannabis daily for legitimate reasons (chronic pain, anxiety, sleep) may have detectable THC in their blood around the clock, even when they're fully clear-headed.

That's not a loophole in the law. It's a flaw in how the law was written.

How Washington's DUI limits compare across substances:

What Actually Happens During a Marijuana DUI Stop in Seattle

Unlike alcohol, there's no roadside breathalyzer for THC. Officers rely on field sobriety tests (walk-and-turn, one-leg stand, follow-the-object eye test) that weren't designed with cannabis impairment in mind. If an officer suspects impairment, they can call in a Drug Recognition Expert (DRE) and request a blood draw.

That blood draw is where the 5 ng/mL threshold kicks in: specifically, the result within two hours of when you were driving. But here's what many people don't know: you have the right to refuse a blood draw. Refusing carries its own consequences under Washington's implied consent law, including a license suspension of at least a year, but it also eliminates the chemical evidence the prosecution typically relies on.

The decision to comply or refuse is not simple. It depends on your usage patterns, when you last used, and the specifics of your stop. This is exactly the kind of call that should involve an attorney before you make it.

How a Marijuana DUI Can Be Defended

A marijuana DUI charge is not a foregone conclusion. Depending on the facts of your case, an attorney may be able to challenge:

- The stop itself: whether the officer had legal grounds to pull you over

- The field sobriety tests, which were designed for alcohol, not cannabis

- The blood draw: whether it was administered correctly and within the required timeframe

- The THC result: through expert testimony on tolerance, impairment, and the science behind the 5 ng/mL limit

Every case is different. But the combination of contested science, the absence of a reliable field test, and Washington's rigid numerical threshold creates more room to fight these charges than most people expect.

Talk to a Seattle DUI Defense Attorney Today

A marijuana DUI doesn't have to define what comes next. At Lewis and Laws, we defend Seattle clients against DUI charges, including cases where the stop was questionable, the test results don't tell the whole story, or the law simply didn't account for your situation. Call us today. The sooner you get legal advice, the more options you have.