HALIFAX—The results of a pilot study that measures how much cannabis has moved through human bodies and been flushed down the toilet add to the evidence that Haligonians consume more cannabis than other Canadians.

Statistics Canada is turning to new methods of measuring cannabis consumption after noting that people are likely to under-report pot use because of stigma and a reluctance to divulge black-market purchases.

Testing for chemical components of cannabis that have gone down the drain could help to paint a more accurate picture, the organization says.

Using a technique called wastewater-based epidemiology (WBE), Statistics Canada says it has estimated the total mass of cannabis consumed in Halifax, Montreal, Toronto, Edmonton, and Vancouver from March to August 2018.

A report released Thursday shows that Halifax surpassed each of the four other cities in the average amount of cannabis consumed per person, per week, at about 1,300 micrograms.

The results came after researchers sampled sewers for one week out of each the study’s six months and tested for a variant of THC — the psychoactive component of cannabis — that had been slightly altered through human metabolism. The waste water plants that were tested are used by about 8.4 million Canadians.

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The error intervals are high and Statistics Canada says the results are “preliminary and experimental,” but the agency says it wants to keep working with the method.

Although the accuracy is limited, the new results reflect the results of the National Cannabis Survey, which Statistics Canada launched in 2018. According to answers from a voluntary questionnaire, 20 to 23 per cent of Nova Scotians consumed cannabis in the first three quarters of 2018 — a larger percentage of the population than in any other province.

Officially, Statistics Canada first released estimates of cannabis consumption last year (about 773 tonnes in 2017) using “survey data from multiple sources,” but this new report concedes that those figures are probably low.

“One of the challenges in collecting statistics on cannabis is that there is a risk that people may under-report their consumption of cannabis, at least initially, in surveys conducted by the agency,” the report reads.

The new report says that WBE has been used to test for a variety of drugs in Europe for more than a decade and has the potential to give quick, cost-effective results.

When it comes to cannabis, Statistics Canada says estimating consumption is difficult because the drug comes in different forms and with different potencies of THC, but because of “significant international research underway on how to improve” the method, the organization wants to keep pursuing it.

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Statistics Canada says more accurate testing could help fill in other information gaps about cannabis markets.

“WBE may also be used to estimate the size of the illegal cannabis market following legalization, by allowing Statistics Canada to subtract the volume of legal cannabis retail sales from total consumption as measured through WBE,” the report says.

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