Which illegal drugs are surging in use around the globe? Research are looking at the world’s sewage for an answer.

In a new study analyzing wastewater published in the journal Addiction, researchers concluded that taking a look at raw sewage can help them assess a city’s overall illicit drug use by yielding crucial data on combined doses per day.

According to findings from the study, cocaine use increased throughout Europe, while methamphetamine use was most common in North America and Australia.

MDMA, also known as ecstasy or molly, had a higher use in the Netherlands.

The analysis concludes that by examining wastewater, samples can serve as an indicator to complement other ways of gauging drug use and help law and drug enforcement agencies determine if their control measures are effective.

The seven year study involved the catchment and analysis of sewage in 37 countries, at more than 140 wastewater treatment plants in 120 cities.

Samples of raw wastewater were gathered from 2011 to 2017, over one week during each of those years.