Last verified: March 2026

How Washington's Cannabis DUI Law Works

Cannabis DUI in Washington is governed by RCW 46.61.502. Washington uses a per se standard, meaning that if your blood contains 5 nanograms per milliliter (ng/mL) or more of active THC (delta-9) within two hours of driving, you are legally impaired — regardless of whether you actually appear impaired.

This is similar to the 0.08% blood alcohol concentration (BAC) standard for alcohol. Prosecutors do not need to prove you were actually impaired; exceeding the THC threshold is sufficient for conviction.

A person is guilty of driving under the influence if the person drives a vehicle within this state while the person has a THC concentration of 5.00 nanograms per milliliter of whole blood or higher within two hours of driving.

RCW 46.61.502 — Driving Under the Influence

The 5 ng/mL Per Se Limit

Key details about Washington's THC blood limit:

• 5 ng/mL of active delta-9-THC in whole blood within 2 hours of driving
• The measurement is taken from a blood draw, not a saliva or urine test
• The test must occur within two hours of the time the person was driving
• Under 21: Any detectable amount of THC in the blood is illegal — zero tolerance

DUI Penalty Chart

First Offense (Gross Misdemeanor)

Second Offense

Third Offense Within 10 Years (Felony)

Medical Authorization Is NOT a Defense

This is critical: having a valid medical cannabis authorization or recognition card provides absolutely no defense against a DUI charge. Even if you are a registered medical patient who uses cannabis legally to treat a qualifying condition, you are subject to the same 5 ng/mL per se limit and the same DUI penalties as any other driver.

The law makes no distinction between recreational and medical users when it comes to impaired driving. If you are at or above 5 ng/mL of active THC in your blood, you are legally impaired regardless of your medical status.

Under 21: Zero Tolerance

Drivers under 21 face a zero-tolerance standard. Any detectable level of THC in the blood is sufficient for a DUI charge, even trace amounts well below the 5 ng/mL adult threshold. This parallels the zero-tolerance approach to underage alcohol DUI.

Cannabis in Your Vehicle: Sealed Container Rule

When transporting cannabis in a vehicle, Washington law requires that it be stored properly:

• Cannabis must be in its original sealed container from the retail store
• It should be stored in the trunk or an area not accessible to the driver or passengers
• Open or previously opened cannabis containers in the passenger area can create legal issues, similar to open container laws for alcohol
• Consuming cannabis inside a vehicle is illegal — even for passengers, even in a parked vehicle

How Cannabis DUI Differs from Alcohol DUI

There are important practical differences between alcohol and cannabis DUI enforcement:

• No roadside breathalyzer equivalent. THC levels are measured through a blood draw, which requires medical personnel and takes time. There is no instant roadside THC test.
• THC metabolites linger. Active THC levels can remain above 5 ng/mL for hours after consumption, especially for daily users. Regular consumers may have elevated baseline THC levels even when not acutely impaired.
• Drug Recognition Experts (DREs). Officers trained as DREs conduct a series of physical and cognitive tests to assess cannabis impairment. Their observations are used alongside blood test results.
• Implied consent. By driving on Washington roads, you consent to chemical testing if an officer has reasonable grounds to believe you are impaired. Refusing a test triggers separate penalties.

Practical Tips for Cannabis Users

• Use rideshare services. Uber, Lyft, and taxis are available throughout the Seattle metro area and other Washington cities. This eliminates all DUI risk.
• Wait before driving. THC impairment from smoking typically peaks within 15–30 minutes and can last 2–4 hours. Edibles can impair for 6–8 hours or longer.
• Be aware of cumulative use. Regular users may maintain THC blood levels above 5 ng/mL even hours after their last use. Plan accordingly.
• Store cannabis properly. Keep all products in original sealed containers in the trunk during transport.
• Never consume in a vehicle. Not while driving, not as a passenger, not even in a parked car.

For more on how cannabis affects driving ability, see Driving & Impairment on TryCannabis.org.

Official Sources

• RCW 46.61.502 — Driving Under the Influence
• Washington State Liquor and Cannabis Board