2.2.1.1 Smoked cannabis The estimation of dose administered by the smoking route is a major variable in the assessment of absorption of cannabinoids (mainly THC) in humans. The source of the plant material and the composition of the cigarette, together with the efficiency of smoking by the subject, are additional uncontrolled factors. It might be reasonable to consider about 10% (range 3–30%) as an average for THC content in Canadian marihuana. It appears that habitual (heavy) marihuana smokers can increase the amount absorbed, which is attributed to more efficient smoking techniques (Agurell, et al., 1986). THC absorption by inhalation is extremely rapid, with a bioavailability of 18 to 50% from the cigarette (Huestis, 1999), and is the main reason this route is preferred by many people (Iverson, 2000, p 46–47). Standardised cigarettes have been developed by NIDA, and the relationships among cannabis (THC) content, dose administered and resultant plasma levels have been investigated. Smoking cannabis containing 1.64% THC (mean dose 13.0 mg THC) resulted in mean peak THC plasma levels of 77 ng/mL (Ohlsson, et al., 1980). THC levels in plasma decreased rapidly after cessation of smoking and were below 5 ng/mL, 2 hours after smoking; mean concentrations declined by about 50%, 15 minutes after (Huestis, et al., 1992) reaching the maximum (Huestis, 1999) . However, THC from a single dose can be detected in plasma for at least a day using modern sensitive analytical techniques and for 13 days in chronic users (Johansson, et al., 1988). The decline of THC in plasma is multiphasic and as Harvey (Harvey, 1999) notes, the estimates of the terminal half-life of THC in humans have increased as analytical methods have become more sensitive. There is still no consensus. It is probably safe to say that the terminal half-life of THC averages at least a week and could be considerably longer. The half-life in plasma does not appear to be different between heavy and light users (Agurell, et al., 1971). Source