Study: Elevated THC Blood Levels Persist for Days in Habitual Cannabis Consumers

Study: Elevated THC Blood Levels Persist for Days in Habitual Cannabis Consumers

By Paul Armentano, NORML

VICTORIA, AUSTRALIA — The presence of THC may persist in the blood of habitual cannabis consumers for multiple days at concentrations above 5ng/ml, according clinical data recently published in the journal Forensic Science International.

Australian researchers assessed daily concentrations of THC in the blood of 21 subjects over a period of 7 days of monitored abstinence. Subjects reported having engaging in the “heavy” use of cannabis during the months leading up to the study.

Of the 11 participants who completed the entire 7-day program, seven tested positive for THC/blood concentrations of 2ng/ml or higher on the final day of the trial. Nine of the 21 subjects tested positive for concentrations of THC above 5ng/ml some 24-hours after their last reported use of cannabis, with one subject testing positive above this threshold some 129 hours after being admitted into the study.

Several other participants experienced rapid declines in THC/blood levels followed by subsequent spikes in THC concentrations following multiple days of cannabis abstinence.

Researchers reported: “While many of the subjects showed expected blood THC profiles, there were several who appeared to have a prolonged redistribution phase lasting a day or more and some who sustained high blood THC concentrations for several days. There were also subjects who showed a ‘double hump’ pattern with an initial fall in levels followed by a transient rise on the third or fourth day of abstinence.”

They concluded: “These results suggest that the toxicokinetics of THC are not as simple as was previously thought. … This makes interpretation of toxicology results much more difficult than it has been when it was assumed that THC followed a well-defined pattern of elimination kinetics and further suggests that a reliable algorithm for mathematical modeling of THC metabolism in real-world heavy users remains elusive.”

The results challenge the notion the presence of THC in blood can be a consistent or accurate predictor of recent cannabis use or behavioral impairment. In particular, these findings further call into question the imposition of per se traffic safety laws for cannabis.

Such laws, which are presently imposed in over a dozen states, legally define drivers with trace levels – or in some cases, any levels – of THC in their blood as criminally impaired.

Full text of the study, “Residual cannabis levels in blood, urine and oral fluid following heavy cannabis use,” appears in Forensic Science Journal. Additional information is available from the NORML website here: http://norml.org/library/driving-and-marijuana.