Last verified: March 2026

No Per Se THC Limit

Missouri is one of the minority of legal states that does not set a specific THC blood concentration threshold for driving impairment. Under §577.010 of the Missouri Revised Statutes, it is illegal to operate a vehicle while "intoxicated" or "in an intoxicated condition" — but the law does not define a numerical THC limit the way it defines 0.08% BAC for alcohol.

This means that the mere presence of THC in your blood does not automatically equal a DUI. The prosecution must prove that you were actually impaired while driving, not simply that you had THC in your system.

The mere presence of any amount of a controlled substance in a person's system does not necessarily indicate that a person is intoxicated or that the person's ability to operate a motor vehicle is impaired.

Missouri Constitution, Article XIV, Section 2 (Amendment 3)

The Constitutional Shield

This is the most remarkable aspect of Missouri's cannabis DUI framework. Amendment 3 itself, now part of the Missouri Constitution, explicitly states that the mere presence of THC "does not necessarily indicate that a person is intoxicated." Legal scholars have argued that this language may constitutionally prevent the legislature from ever imposing a per se THC threshold.

Consider the implications:

• In states like Michigan, any THC above 1 ng/mL is a per se violation — you are guilty regardless of actual impairment
• In states like Colorado, 5 ng/mL creates a "permissible inference" of impairment
• In Missouri, the Constitution itself says THC presence alone does not prove intoxication. A per se threshold would directly contradict this language

No court has definitively ruled on whether Amendment 3 bars a per se THC law, but the constitutional language is remarkably clear and creates a strong defense for any future legal challenge.

How Cannabis DUI Is Enforced in Missouri

Without a per se THC limit, Missouri relies on a Drug Recognition Expert (DRE) based enforcement model:

Step 1: The Traffic Stop

An officer must have reasonable suspicion to initiate a stop (swerving, speeding, running a light, etc.). The smell of cannabis alone may provide probable cause for further investigation, but Amendment 3's protections make this more nuanced than before legalization.

Step 2: Field Sobriety Tests

Standard field sobriety tests (walk-and-turn, one-leg stand, horizontal gaze nystagmus) are designed primarily for alcohol impairment and are less reliable for detecting cannabis impairment. Officers will note observations like bloodshot eyes, slowed speech, or impaired coordination.

Step 3: Drug Recognition Expert (DRE) Evaluation

If the officer suspects drug impairment, a DRE-certified officer may conduct a 12-step evaluation protocol. This is the primary tool for cannabis DUI enforcement in Missouri. However, there is a significant limitation:

With approximately 1% of Missouri law enforcement officers holding DRE certification, many departments simply do not have a DRE available during a given shift. This creates practical enforcement gaps, particularly in rural areas and smaller departments.

Step 4: Chemical Testing

A blood or urine test may be requested to confirm the presence of THC. However, under Missouri's constitutional framework, a positive THC test alone is not sufficient for conviction. The test may corroborate other evidence of impairment, but it cannot be the sole basis for a DUI charge.

Implied Consent

Missouri has an implied consent law. By driving on Missouri roads, you have implicitly consented to chemical testing if an officer has probable cause to believe you are impaired. Refusing a test triggers automatic consequences:

• 1-year license revocation for refusal
• The refusal can be used against you in court
• The revocation applies regardless of whether you are ultimately convicted of DUI

Penalties for Cannabis DUI

Practical Considerations

• THC stays in your system long after impairment ends. Regular users can test positive for days or weeks after last use. Missouri's lack of a per se threshold protects you from being convicted based on lingering THC alone — but a positive test combined with bad driving is still dangerous territory.
• Do not drive impaired. Regardless of the legal framework, cannabis impairs reaction time, attention, and decision-making. If you have consumed recently, use a rideshare or designated driver.
• Keep cannabis sealed in the vehicle. Open containers in the passenger compartment create problems similar to open alcohol containers and give officers additional grounds for investigation.
• Know your rights. You can decline field sobriety tests (they are voluntary), but refusing a chemical test triggers the 1-year implied consent revocation.

Official Sources

• Missouri Revised Statutes §577.010 (DWI)
• Missouri Constitution, Article XIV, Section 2
• Division of Cannabis Regulation (DCR)