Scientists spent years screening sewage from some 60 million people for cocaine, amphetamines, and MDMA to build a comprehensive wastewater map of illicit drug use in more than 30 countries around the world.

According to the study, which was published Wednesday in the journal Addiction, cocaine use is on the rise across Europe, methamphetamine is most prevalent in North America and Australasia, and the Netherlands had the highest rate of MDMA use.

“This is the largest wastewater‐based epidemiology study ever performed in terms of cities (120) and countries (37) involved and of the monitoring duration (2011–17),” said the team, which was co-led by Iria González-Mariño, an assistant professor of chemistry at the University of Salamanca in Spain.

“The extensive data set obtained for cocaine, amphetamine, methamphetamine, and MDMA showed a comprehensive picture of spatial and temporal trends of use,” González-Mariño and her colleagues said in the study.

Sampling wastewater for residue from drugs has become an increasingly popular method to track patterns in the global illicit drug market over the past decade. Previous studies have focused on drug use in select cities or during public events, such as the 2017 total solar eclipse, but González-Mariño’s and her colleagues took an international approach.

The team began sampling wastewater in European cities in 2011, and expanded the survey to Australia, New Zealand, Colombia, Martinique, Canada, the US, South Korea, and Israel from 2014 to 2017.

The team avoided collecting samples during public holidays, when higher-than-average drug use might skew the results, and tried to account for contamination of prescribed drugs that might leave the same chemical fingerprints as their illegal variants. For instance, Seattle wastewater suggests especially high methamphetamine use in the city, but this finding could be influenced by legal drugs prescribed for attention deficit/hyperactivity disorders.

The results showed that overall drug use was most prevalent cities such as Antwerp, Amsterdam, Zurich, London, and Barcelona, while cities in Greece, Portugal, Finland, Poland, and Sweden had the lowest rates of drug residue in wastewater.

Cocaine was most popular in London, Bristol, Amsterdam, Zurich, Geneva, St Gallen, and Antwerp. Amphetamine seemed to be on the rise in Barcelona, Geneva, Berne, Zurich, Dortmund, and Berlin, but methamphetamine use was far lower in Europe than in the United States and Australia. While the Netherlands had the highest rates of MDMA use, the drug was also popular in Helsinki, Oslo, Brussels, Dortmund, Zagreb, Zurich, Geneva, and Barcelona.

Overall, the data more or less lined up with other measurements of illicit drug use. “In general terms, conclusions we got by analyzing wastewater match the ones derived from established drug use indicators like surveys, incaution data, etc.,” González-Mariño said in an email. “The interesting thing of our methodology is that it provides data almost in real time (samples are taken and analyzed within few days) so we get results ‘earlier’ than other indicators.”

As an example, she recalled how her team was perplexed to see a spike in benzoylecgonine, the metabolite compound in cocaine, in European cities from 2016 to 2017. Months later, the researchers found out that cocaine purity had increased during that period, which could account for the higher levels of benzoylecgonine in the wastewater.

González-Mariño also sampled wastewater for signs of cannabis use, but that drug’s psychoactive ingredient— tetrahydrocannabinol (THC)—was more difficult to identify in sewage samples because it easily bonds with wastewater particles.

“The analytical protocol to extract it and analyze it in water had to be improved year by year,” González-Mariño said. “It is a tricky substance but I am confident our new methods will be good enough to include THC also in our next studies.”

This wastewater data is useful for communities aiming to track the flow of illicit drugs, and anticipating the medical and social problems interlinked with them. González-Mariño and her colleagues hope to expand their research to other cities, and potentially start testing for other substances that affect human quality of life.