Can Regular Marijuana Use Keep You Over Washington’s THC Limit?

Many Washington drivers assume that if they feel sober, they are safe to drive. But for frequent cannabis users, that may not always be the case.

Even after the intoxicating effects of marijuana have faded, residual THC in the bloodstream can sometimes exceed Washington’s legal limit for driving. Below, our DUI defense attorneys explain what the law says, how THC testing works, and why regular users should be especially cautious before getting behind the wheel.

For help with your case, contact the Law Offices of Smith & White at (253) 203-1645.

Washington’s 5 ng/mL Standard

Under RCW 46.61.502, it is illegal in Washington to drive either:

while impaired by alcohol, cannabis, or any drug, or
with a THC concentration of 5.00 nanograms per milliliter (ng/mL) or more in whole blood within two hours of driving.

This 5 ng/mL limit—often called a per se standard—means that a driver can be charged with DUI based solely on the blood test result, even if there are no visible signs of impairment.

How Prosecutors Use THC Test Results

If a blood sample is collected more than two hours after driving, RCW 46.61.502(4)(b) allows the result to be used as evidence of what the level likely was during that two-hour window.

Meanwhile, RCW 46.61.506 governs how test results are admitted in court. It makes clear that blood tests are not the only evidence that matters—officer observations, driving behavior, and field sobriety performance can also be used to prove or disprove impairment.

This framework gives prosecutors flexibility, but it also provides opportunities for defense attorneys to challenge how test results were collected, interpreted, or applied.

Why THC Blood Tests Don’t Always Prove Impairment

THC behaves differently from alcohol, and that difference can make blood test results misleading.

Fat-soluble behavior: THC moves quickly from the bloodstream into fatty tissues, then slowly releases back over time.
Rapid decline: After smoking or vaping, THC levels in whole blood can drop by 80–90% within the first hour, depending on dose and metabolism.
Residual presence: Regular users often have low but steady THC levels that remain detectable long after the “high” is gone—sometimes above 5 ng/mL even after 12 hours or more of abstinence.
Weak link to impairment: The National Highway Traffic Safety Administration’s 2017 report, Marijuana-Impaired Driving: A Report to Congress, found that blood THC concentration does not reliably predict crash risk or impairment once tolerance and individual variation are considered.

Because of these biological factors, a THC number alone rarely tells the full story of a driver’s condition at the time of driving.

These differences matter most for frequent cannabis users, whose systems handle THC differently over time.

How Long Does THC Stay Detectable — and When Is It Safe to Drive?

Washington law does not define a “safe” waiting period after marijuana use, but scientific research and public health guidance offer general recommendations:

Smoking or vaping: wait at least 6–8 hours before driving.
Edibles or tinctures: wait 8–12 hours, since effects last longer.
Frequent users: residual THC may linger far longer—some individuals still test above 5 ng/mL the following day even when sober.
Cannabis combined with alcohol: impairment increases dramatically, so waiting longer is strongly advised.

These are guidelines, not legal limits, but they show why it’s possible to test over the legal threshold even when you no longer feel impaired.

Special Rules for Drivers Under 21

Drivers under age 21 face stricter limits under RCW 46.61.503.

Any THC level above 0.00 ng/mL but below 5.00 ng/mL within two hours of driving can result in a “Driver Under 21 Consuming Alcohol or Cannabis” charge, often called a Minor DUI.
If the THC level is 5 ng/mL or higher, prosecutors can instead file a standard adult DUI under RCW 46.61.502.

In State v. Fraser (2022), the Washington Supreme Court upheld this structure, confirming the constitutionality of the 5 ng/mL limit established by Initiative 502.

Example Scenario

Marisol uses a small amount of cannabis each night before bed. The next morning, she drives to work feeling completely alert. On her commute, she is pulled over for a minor traffic infraction. During the stop, the officer detects a faint odor of marijuana and requests a blood test. Ninety minutes later, the result shows 5.4 ng/mL THC.

Prosecution’s position: The reading and timing support a per se DUI charge.
Defense argument: The result likely reflects residual THC from prior use, not active intoxication, and Marisol showed no signs of impairment.

Cases like this are common in Washington. They often turn on timing, test accuracy, and expert interpretation, not just the number on a lab report.

Frequently Asked Questions

Can I be charged if I’m not high anymore?
Yes. A test showing 5 ng/mL or more within two hours of driving can support a DUI charge. A defense can argue that the level reflects residual THC, not impairment.

If I use marijuana regularly, will I always test over the limit?
Not necessarily. It depends on your body chemistry, frequency of use, and the time since your last use. Some regular users remain above 5 ng/mL longer than others.

Does Washington recognize that THC numbers may not equal impairment?
Yes. RCW 46.61.506 allows “other competent evidence,” and national research, including NHTSA’s 2017 report, supports that THC concentration alone is not a reliable measure of impairment.

How long should I wait before driving?
There’s no fixed rule. Waiting 6–8 hours after smoking or 8–12 hours after edibles is generally considered safer, especially for frequent users.

What if I’m under 21?
Drivers under 21 are held to a zero-tolerance rule. Any THC above 0.00 ng/mL can lead to a Minor DUI, while 5 ng/mL or higher can trigger a standard DUI charge.

What to Do Next If You’re Facing a THC DUI

A marijuana-related DUI charge does not automatically mean conviction. THC evidence can be challenged based on timing, lab procedure, and scientific reliability.

If you’ve been arrested for a suspected THC DUI:

Don’t assume the test result is final—it can be contested.
Request copies of your lab and arrest reports for review.
Act quickly to request a Department of Licensing (DOL) hearing to protect your license.
Contact an experienced DUI defense attorney familiar with marijuana-related cases.

An attorney who understands both the law and the science of THC testing can make a significant difference in how your case is resolved.

Bottom Line

Regular cannabis use does not guarantee you’ll exceed Washington’s 5 ng/mL THC limit, but it does increase the risk—especially for frequent users.

Because THC behaves unpredictably in the body, DUI cases involving marijuana often depend on timing, context, and expert evidence, not just a number.

If you are charged with a marijuana-related DUI, the attorneys at the Law Offices of Smith & White can help. We carefully review every detail—from lab procedures to officer observations—to build a defense grounded in both law and science.

Call (253) 203-1645 today for a confidential consultation.