What a whole city flushes: the science of testing sewage for drugs

Your city's sewer is the most honest survey it never agreed to take. Every day, with no questionnaire and no names, it records roughly how much cocaine, MDMA, and amphetamine a whole population used the night before. Scientists have learned to read it, and the field even has a name: wastewater-based epidemiology.

How wastewater drug testing works

The method is straightforward in principle. Researchers collect samples of untreated sewage as it flows into a treatment plant and measure the concentrations of drugs and their metabolites — the breakdown products a body excretes after use. Knowing how many people the plant serves, they can back-calculate an estimate of how much of a drug the community consumed. No individual is ever identified; the sewer only speaks in totals.

The largest experiment of its kind

Since 2011, a network of scientists called SCORE — the Sewage analysis CORe group Europe — has run an annual multi-city study in partnership with the European Union's drugs agency. It has grown into the largest project of its kind in the world; the most recent round sampled the wastewater of 88 European cities, turning a continent's plumbing into a shared dataset.

What the sewers say

The 2024 findings put cocaine centre stage: detections rose across dozens of cities, continuing an upward trend seen since 2016 — highest in western Europe but climbing in the east. The sewer, it turns out, has been keeping receipts. Because samples can be taken day by day, the approach can also catch a spike within days, far faster than a traditional survey.

Why we care about what is in the water

This is the mirror image of a story we have told before. The same drugs that show up in a city's sewage also slip through treatment and into rivers and wildlife downstream — the phenomenon behind the “cocaine sharks” headlines. Wastewater analysis turns that leak into something useful: an anonymous, near-real-time public-health mirror. It will never tell you who; it tells you how much — and sometimes that is exactly the number that matters.