In the fall of 2018, Canada legalized cannabis for recreational use. Given this major change in policy, Statistics Canada wanted to update and improve their estimates of cannabis consumption. However, estimating drug use with traditional surveys presents a challenge because respondents are likely to under-report their consumption.
To supplement surveys, Statistics Canada piloted the emerging science of wastewater-based epidemiology (WBE), which involves measuring wastewater in the sewers for trace concentrations of a cannabis metabolite that is created and excreted after consuming cannabis. This same approach can also be applied to other drugs of interest, including methamphetamine, cocaine, and opioids. In this paper, we validate the WBE measurements and evaluate their utility for estimating various quantities relating to societal drug use.
WBE is particularly well-suited to direct comparisons of the wastewater at different times or locations. It detected large temporal patterns for some of the drugs, including cannabis. It also identified geographical patterns of drug consumption that were distinct for each drug, revealing different drug-use profiles for the five cities in the study.
However, WBE estimates of annual drug consumption were of mixed quality, depending on the drug. Methamphetamine and cocaine consumption estimates were reasonably precise. The cannabis estimate was more variable, primarily due to uncertainty in the amount of cannabis metabolite excreted after consuming cannabis. Estimates of individual opioids were not attempted at this time because too many distinct drugs leave the same trace in the wastewater, making them impossible to distinguish.
This project demonstrated that wastewater contains information about societal drug use. While WBE does have some limitations that should be evaluated when considering a specific application, it could provide a valuable source of information when used in concert with surveys.