An international working group of 11 experts from six countries suggests a limit of 7-10 ng/mL THC in blood serum for driving under the influence of cannabis. This would compare to a blood alcohol concentration of about 0.05 per cent. The group states that zero-limit laws, which are in effect in several countries and in several states of the USA, are not science-based and classify many non-impaired drivers as being under the influence of cannabis. (Source: Grotenhermen F. et al. Addiction 2007 Oct 4; [Electronic publication ahead of print])