Drug testers sifting through raw sewage in 19 European cities found the highest cocaine use in Antwerp, a Nordic preference for methamphetamines and Amsterdam unsurprisingly leading in cannabis use.
From the biggest-ever European drug analysis of human waste samples, the research team deducted that the continent used about 350 kilograms of cocaine every day while marijuana remained the most popular illicit drug.
"Through research into the sewer, we can determine how big the drug market in a city is," coordinator Kevin Thomas of the Norwegian Institute for Water Research said of the study covering cities in 11 European countries.
The team took samples from the inlets of 21 sewage treatment plants servicing a combined population of some 15 million people -- about two percent of the continent's population.
The samples were taken on seven consecutive days from March 9, 2011, and then analysed in the lab.
The results published Thursday in the journal Science in the Total Environment revealed the highest average cocaine use in the Belgian port city of Antwerp followed by Amsterdam (Netherlands), Valencia (Spain), Eindhoven (Netherlands), Barcelona (Spain), London (UK), Castellon (Spain) and Utrecht (Netherlands).
The Dutch cities of Amsterdam, Utrecht and Eindhoven showed the highest sewage loads of the nightclubber's drug ecstasy, though the authors said Utrecht's figures were probably spiked by drug dumping in a police raid on an ecstasy factory two days prior to the study.
Antwerp and London also had high levels of ecstasy use, but none was detected in Castellon, Umea (Sweden) or Stockholm (Sweden).
The study said the highest methamphetamine use was measured in Helsinki and Turku in Finland, Oslo (Norway) and Budweis in the Czech Republic.
Methamphetamine, also known as "crystal meth", "glass" or "ice", is a stimulant that is relatively cheap and easy to produce.
Illicit use of the related compound amphetamine or "speed", originally used to treat some medical conditions, was highest in Eindhoven and Antwerp.
For cannabis, the highest levels by far were measured in the Dutch capital, which is a popular soft-drug tourist destination in a country that tolerates and regulates marijuana use.
About seven percent of the European population aged 15 to 64 used cannabis, according to the report, with Paris the second biggest user in the study.
The researchers found drug use spiking on Friday and Saturday nights.
The report cited earlier findings that about a third of European citizens have tried an illicit drug. At least one person dies of an overdose every hour.
Yet monitoring illicit drug use is difficult as people are prone to lying in surveys for fear of being caught out -- leaving drug controllers without much-needed data.
Studies like these "can provide a diluted and pooled community urine sample that can deliver objective near real-time estimates of the total quantities of illicit drugs being used," said the report.
"We are not there to replace the police," Sara Karolak of France's Paris Sud university, which took part in the survey, told AFP.
"We want this method to be used as a prevention tool," she said, adding that a new study has already started, with more countries.