Saturday, 18 June 2016

Stoned driving

It's hard to get a good read on the risks of driving while stoned. It matters because it's one of the less crazy objections to marijuana legalisation: if penalising driving while stoned is hard, and if it is risky, and if consumption increases under legalisation, that could be a cost of legalisation.

Existing estimates are a bit of a problem. Studies finding associations can fail to account properly for alcohol use, or for that cannabis in the system does not necessarily indicate recent use. There are blood tests that can more reliably indicate acute intoxication, and saliva tests, but urine tests don't say much about recent use.

Ole Rogeberg and Rune Elvik look back at a couple of meta-studies and tries to make sense of things. They go back and properly account for how different studies measured drug use, what other confounding factors were included, and whether there were any dose-response relationship established.

They find current metastudies overestimate the effects of cannabis on crash risk. They note that the relative risk of driving at 0.05 BAC is 3.6. Controlling for alcohol use reduces the relative risk of cannabis use from about 1.7 to about 1.2.

They conclude:
A comprehensive review of the literature on acute cannabis intoxication and road traffic crashes finds that acute intoxication is related to a statistically significant risk increase of low to moderate magnitude. Higher estimates from earlier meta-reviews were found to be driven largely by methodological issues - in particular, the use of counts data without adjustment for known confounders. Correcting for these issues, the pooled estimates from these reviews were in line with the results from the updated and more extensive review.
How big is a relative risk of either 1.7 or 1.2?
A few "driving while" risks: 
It is frustrating to watch America legalise cannabis, state-by-state, and the UK debate whether legalisation or decriminalisation is best for all drugs, while NZ remains stuck in the stone age.

A comprehensive review of the literature on acute canna-
bis intoxication and road trafc crashes nds that acute
intoxication is related to a statistically signicant risk
increase of low to moderate magnitude. Higher estimates
from earlier meta-reviews were found to be driven largely
by methodological issuesin particular, the use of counts
data without adjustment for known confounders.
Correcting for these issues, the pooled estimates from these
reviews were in line with the results from the updated and
more extensive review. Remaining selection effects
discussed in the Alternative interpretations section may
complicate causal interpretations of the pooled estimates.
A comprehensive review of the literature on acute canna-
bis intoxication and road trafc crashes nds that acute
intoxication is related to a statistically signicant risk
increase of low to moderate magnitude. Higher estimates
from earlier meta-reviews were found to be driven largely
by methodological issuesin particular, the use of counts
data without adjustment for known confounders.
Correcting for these issues, the pooled estimates from these
reviews were in line with the results from the updated and
more extensive review. Remaining selection effects
discussed in the Alternative interpretations section may
complicate causal interpretations of the pooled estimates.

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