Scientists say the drugs pass through people and then the wastewater is treated, before being dumped in reservoirs, rivers or lakes.
The city of Fresno doesn't test for prescription drugs and isn't required to. But officials say the risk is extremely low.
Lon Martin, Fresno Dept. Of Public Utilities: "At first it can seem scary, but when you actually look at quantities in drinking water, it's extremely small amounts."
Fresno did test its wastewater treatment facility a few years ago and water officials say no prescription drugs were found.
They expect the state in the near future to start requiring testing for drugs in the tap water.