Study shows that frequent users may exceed legal THC limits even after abstinence
Research reveals that THC concentrations above the thresholds used in driving under the influence laws can remain in the blood days after the last use, without indicating impairment in driving ability
A study published in the Clinical Chemistry analyzed 190 frequent cannabis users who remained abstinent for at least 48 hours. The researchers found that 43% of participants exceeded the zero tolerance limit (≥0.5 ng/mL of THC in blood), while 24% surpassed the cutoff of 2 ng/mL 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 assessed 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, averaging 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.