A study published in Clinical Chemistry analyzed 190 frequent cannabis users who remained at least 48 hours without consuming the substance. Researchers identified that 43% of the participants exceeded the zero tolerance limit (≥0.5 ng/mL of THC in the blood), while 24% surpassed the 2 ng/mL cutoff and 5.3% were above 5 ng/mL — values used in per se laws in the United States. In some cases, concentrations reached 16.2 ng/mL even after two days of abstinence.

The team also evaluated the participants' performance in a driving simulator and found no significant difference between those above the legal limits and those below them. After a controlled consumption session, THC levels increased modestly, with an average of 0.5 ng/mL above baseline after six hours.

According to the authors, the study reinforces that blood THC concentrations do not necessarily correspond to impairment in driving, which may have legal implications for regular users who are not under the influence at the time of driving.

Source: Fitzgerald RL et al. Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence. Clinical Chemistry, 2025. PMID: 41222016.