Colorado Marijuana DUI Defense: Blood THC Levels Are Not Proof of Impairment

Colorado treats alcohol and THC evidence differently. An alcohol concentration at or above the statutory level can establish a per se alcohol offense, but a THC result does not operate as an identical automatic limit. In a marijuana case, the prosecution still must prove the charged impairment element beyond a reasonable doubt using the totality of the evidence.

Under C.R.S. § 42-4-1301(6), five nanograms or more of delta-9 THC per milliliter of whole blood permits the factfinder to infer that the driver was under the influence of one or more drugs. The inference is permissive, not mandatory. Jurors may accept or reject it after considering driving, observations, testing conditions, admissions, toxicology, and alternative explanations.

THC pharmacokinetics make a simple number-to-impairment comparison difficult. Concentrations can rise and fall rapidly after use, and the time between driving and blood collection matters. Frequent users may retain measurable THC or metabolites and can present a different relationship between concentration and observable impairment. A metabolite result also should not be treated as though it were active THC without scientific support.

A defense toxicologist can explain specimen type, collection and storage, laboratory uncertainty, timing, active THC versus metabolites, tolerance, and the limited ability to extrapolate backward to the time of driving. Counsel should obtain chromatograms, calibration records, quality-control data, chain-of-custody documents, and the laboratory’s methods before deciding whether an independent retest or admissibility challenge is warranted.

Standardized field sobriety tests were validated primarily in the alcohol context and do not measure THC concentration. Performance may be affected by medical conditions, fatigue, anxiety, weather, footwear, surface conditions, instructions, or officer administration. Body-camera footage can be more informative than conclusory report language, particularly when it shows normal speech, balance, divided attention, navigation, and interaction.

The prosecution ordinarily relies on more than the blood result: driving behavior, collision evidence, odor, statements, recent-use indicators, divided-attention performance, and observations by trained officers may all be offered. Defense counsel should scrutinize any jury instruction to ensure the five-nanogram inference remains permissive and does not shift the burden of proof. Current appellate authority should be checked because the wording and evidentiary foundation can determine whether an instruction is proper.