Conversion factors for assessment of driving impairment after exposure to multiple benzodiazepines/z-hypnotics or opioids.

AIMS: Norway has introduced legal concentration limits in blood for 28 non-alcohol drugs in driving under the influence cases. As of 2016 this legislation also regulates the assessment of combined effects of multiple benzodiazepines and opioids. We herein describe the employed methodology for the equivalence tables for concentrations of benzodiazepines/z-hypnotics and opioids implemented in the Norwegian Road Traffic Act. METHODS: Legislative limits corresponding to impairment at blood alcohol concentrations (BAC) of 0.02%, 0.05% and 0.12% were established for 15 different benzodiazepines and opioids. This was based on a concept of a linear relationship between blood drug concentration and impairment in drug naïve users. Concentration ratios between these drugs were used to establish conversion factors and calculate net impairment using diazepam and morphine equivalents. RESULTS: Conversion factors were established for 14 benzodiazepines/z-hypnotics (alprazolam, bromazepam, clobazam, clonazepam, etizolam, flunitrazepam, lorazepam, nitrazepam, nordiazepam, oxazepam, phenazepam, temazepam, zolpidem and zopiclone) and two opioids (methadone and oxycodone). CONCLUSIONS: Conversion factors to calculate diazepam and morphine equivalents for benzodiazepines/z-hypnotics and selected opioids, respectively, have been operative in the Norwegian Road Traffic Act as of February 2016. Calculated equivalents can be applied by the courts to meter out sanctions.

Impairment based legislative limits for driving under the influence of non-alcohol drugs in Norway.

AIMS: When non-alcohol drugs are detected in blood samples from apprehended drivers in Norway, individualised expert opinions are required to evaluate degree of impairment. For alcohol, legislative limits have been in use since 1936. To harmonize the current practice for driving under the influence of alcohol and non-alcohol drugs, a judicial reform with legislative limits for non-alcohol drugs has been suggested. METHODS: Impairment limits, representing drug concentrations in blood likely to be accompanied by a degree of impairment comparable to a blood alcohol concentration (BAC) of 0.02%, were proposed for 20 psychotropic drugs, including the most prevalent benzodiazepines, cannabis, GHB, hallucinogens and opioids. Limits for graded sanctions, representing drug concentrations in blood likely to induce impairment comparable to BACs of 0.05% and 0.12%, were defined for 13 of the 20 substances. The suggested limits were based on assessments of impairment after single doses of the drugs in naïve individuals. The proposed limits will not apply to individuals with valid prescriptions for medicinal drugs, where the present system with individualised expert evaluations will be maintained. CONCLUSION: Norway is the first country planning to implement legislative limits for non-alcohol drugs corresponding to impairment seen at increasing BACs. The background and justification for the suggested limits are presented herein.