THC in blood:
How long can THC and his metabolites be detected in blood?
With usual tests 0.5 nanograms per milliliter (ng/ml) of THC and 0.5 ng/ml of its metabolite THC-COOH can be detected in blood plasma. The time until falling short of this detection limit after consumption varies considerably, even if the same amount of THC was ingested.
After smoking a low dose cannabis cigarette (about 16 mg THC) the detection limit of 0.5 ng/ml THC in plasma was reached after 7.2 hours on average (range: 3-12 hours) and following a high dose cigarette (about 34 mg THC) a plasma concentration of 0.5 ng/ml THC was reached within 12.5 hours (range: 6-27 hours). The metabolite THC-COOH was detectable for a considerably longer time, for 3,5 days (range: 2-7 days) after the low dose and for 6,3 days (range 3-7 days) after smoking the high dose cigarette.
The elimination half life for THC metabolites from plasma is longer than the elimination half life of the THC itsself. With regular use THC-COOH may be detectable in the plasma for several weeks.
Modified according to: Grotenhermen F. Pharmacokinetics and pharmacodynamics of cannabinoids. Clin Pharmacokin 2002, in press.