You have a headache and take a pill. The headache is gone, but what about the pill? What we flush away makes its way through sewers, treatment works, rivers and streams and finally back to your tap. Along the way most of the drugs we take are removed but the tiny amounts that remain are having effects. Feminised fish in our rivers, starlings feeding on Prozac-rich worms, and bacteria developing antibiotic resistance - scientists are just beginning to understand how the drugs we take are leaving their mark on the environment.

The compounds we excrete are also telling tales on us. Professor of Chemistry, Andrea Sella, gets up close and personal with music festival toilets to find out what the revellers are swallowing, and hears from scientists who are sampling our rivers to learn about our health.

Producer: Lorna Stewart

(Photo: Laboratory technician, testing a urine sample for traces of any banned substances or stimulants. BBC copyright)