Many people choose Prozac over other antidepressants
An Environment Agency report suggests so many people are taking the drug nowadays it is building up in rivers and groundwater.
A report in Sunday's Observer says the government's environment watchdog has discussed the impact for human health.
A spokesman for the Drinking Water Inspectorate (DWI) said the Prozac found was most likely highly diluted.
The newspaper says environmentalists are calling for an urgent investigation into the evidence.
It quotes the Liberal Democrats' environment spokesman, Norman Baker MP, as saying the picture emerging looked like "a case of hidden mass medication upon the unsuspecting public".
He says: "It is alarming that there is no monitoring of levels of Prozac and other pharmacy residues in our drinking water."
Experts say the anti-depression drug gets into the rivers and water system via treated sewage water.
The DWI said the Prozac (known technically as fluoxetine) was unlikely to pose a health risk as it was so "watered down".
The Observer says the revelations raise new fears over how many prescriptions for the drug are given out by doctors.
In the decade leading up to 2001, the number of prescriptions for antidepressants went up from nine million per year to 24 million per year, says the paper.
The Environment Agency report concluded that the Prozac in the water table could be potentially toxic and said the drug was a "potential concern".
The exact amount of Prozac in the nation's drinking water is not known.