"There is more than one way to measure prescription drug use.
The most direct method is to count prescriptions filled by pharmacies. That would show, for example, that more than 180 million prescriptions for diabetes drugs were dispensed in 2013.
Or you could test the treated water coming out of sewage-treatment plants.
That approach reveals that in Lake Michigan waters outside one plant, the diabetes drug metformin was the most common personal-care product found by researchers with the School of Freshwater Sciences at the University of Wisconsin-Milwaukee."