Measuring illegal drug use in a city or country is a challenging task. How do you get reliable data for something that is sold on a black market and hidden from sight? But, several years ago, researchers discovered a roundabout way of estimating an area's drug use—look at its sewage.

Before, researchers had to rely solely on indirect methods, such as looking at police records and asking people about their drug habits, and extrapolate from there. The sewage method, on the other hand, is particularly useful because sewage does not lie. It's a direct way of measuring how much of which drugs a community is excreting, and thus ingesting.

The method was pioneered in 2005, when a group of Italian researchers used it to measure cocaine in sewage and rivers in mid-sized cities across Italy. They found that they could analyze the sludge for both the drug itself and for the metabolites a person's body produces in response to it. Both the river and the sewage contained cocaine. "The largest Italian river, the Po, with a five-million people catchment basin, steadily carried the equivalent of about 4 kg cocaine per day," they wrote. "This would imply an average daily use of at least 27 ± 5 doses (100 mg each) for every 1000 young adults, an estimate that greatly exceeds official national figures."

Since then, the method has been adopted in Europe and the U.S., and expanded to measure not just cocaine but other illegal and prescription drugs as well. In Holland, not surprisingly, researchers found that more urban areas had higher levels of drugs in the sewage compared to rural places or the suburbs. (Except, that is, near the airport: "Methamphetamine was only detected at Schiphol, a fact that was interpreted to be caused by consumption of this drug by travelers.").

In a study conducted across 19 cities in Europe, coke proved more popular in Western and Central Europe compared to Eastern and Northern Europe. Ecstasy loads were particularly hight in Antwerp, London and throughout Holland, whereas methamphetamines spiked in Helsinki, Turku (also in Finland), Oslo and Budweis (Czech Republic). Coke and ecstasy use spiked on weekends, and weed was more or less equally popular across the continent.

Sewage analyses conducted in seven U.S. municipalities found the highest levels of methamphetamines in any study to date, but revealed that cocaine use in the U.S. seems more or less on par with that in Europe.

Finally, the most recent study, conducted throughout Sweden, found evidence of 13 different drugs, including four different hallucinogens, heroin, amphetamines, morphine, oxycodone, plenty of cannabis and significant amount of zolpidem, a drug prescribed for insomnia that tends to cause a "hypnotic state" in those who take it and can produce some hallucinations of its own.

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