Pick up a glass, fill it from the tap and take a sip. You just had a tiny dose of the pill your neighbour took days before. Excreted and flushed through our sewage works and waterways, drug molecules are all around us. A recent analysis of streams in the US detected an entire pharmacy: diabetic medications, muscle relaxants, opioids, antibiotics, antidepressants and more. Drugs have even been found in crops irrigated by treated waste water. The amounts that end up in your glass are minuscule, and will not lay you low tomorrow. However, someone prescribed multiple drugs is more likely to experience side effects due to these small doses, and risks rise exponentially with each drug taken by a person over 65. “These drugs have been individually approved but we have not studied what it means when they are together in the same soup,” says Mae Wu at the National Resources Defense Council (NRDC).