THC and CBD in blood samples and seizures in Norway: Does CBD affect THC-induced impairment in apprehended subjects?

BACKGROUND AND AIMS: Several publications have suggested increasing cannabis potency over the last decade, which, together with lower amounts of cannabidiol (CBD), could contribute to an increase in adverse effects after cannabis smoking. Naturalistic studies on tetrahydrocannabinol (THC) and CBD in blood samples are, however, missing. This study aimed to investigate the relationship between THC- and CBD concentrations in blood samples among cannabis users, and to compare cannabinoid concentrations with the outcome of a clinical test of impairment (CTI) and between traffic accidents and non-accident driving under the influence of drugs (DUID)-cases. Assessment of THC- and CBD contents in cannabis seizures was also included. METHODS: THC- and CBD concentrations in blood samples from subjects apprehended in Norway from April 2013-April 2015 were included (n=6134). A CTI result was compared with analytical findings in cases where only THC and/or CBD were detected (n=705). THC- and CBD content was measured in 41 cannabis seizures. RESULTS: Among THC-positive blood samples, 76% also tested positive for CBD. There was a strong correlation between THC- and CBD concentrations in blood samples (Pearson's r=0.714, p<0.0005). Subjects judged as impaired by a CTI had significantly higher THC- (p<0.001) and CBD (p=0.008) concentrations compared with not impaired subjects, but after multivariate analyses, impairment could only be related to THC concentration (p=0.004). Analyzing seizures revealed THC/CBD ratios of 2:1 for hashish and 200:1 for marijuana. CONCLUSIONS: More than ¾ of the blood samples testing positive for THC, among subjects apprehended in Norway, also tested positive for CBD, suggesting frequent consumption of high CBD cannabis products. The simultaneous presence of CBD in blood does, however, not appear to affect THC-induced impairment on a CTI. Seizure sample analysis did not reveal high potency cannabis products, and while CBD content appeared high in hashish, it was almost absent in marijuana.