A blood concentration of tetrahydrocannabinol (THC) in the low nanograms-per-milliliter range is often claimed to result from drug use more than 24-48 h previously. The present investigation determined concentrations of cannabinoids in blood collected at least 24 h from smoking in an in-patient setting. During sampling, distinctive effects due to drug use could not be observed. The randomly collected samples from heavy (n = 16, > 1 joint/day), moderate (n = 15, < or = 1 joint/day), and light (n = 6, < 1 joint/week) users of cannabis were analyzed for THC, 11-hydroxy-THC (OH-THC), and free 11-nor-9-carboxy-THC (THCCOOH) by gas chromatography-mass spectrometry as well as for glucuronidated THCCOOH by liquid chromatography-tandem mass spectrometry. THC was detectable in 9, 6, and 1 samples from heavy, moderate, and light users, respectively. Although cannabinoid concentrations were overlapping between groups, there was a trend towards higher concentrations of both conjugated and free THCCOOH in regular users compared to occasional users. The present findings appear to indicate that low levels of THC, or of THC along with OH-THC, may not unequivocally prove a very recent use of cannabis.