Tiny quantities of anti-depressants are affecting the fertility of aquatic wildlife such as crustaceans and molluscs, a new study has shown.
Scientists say they are becoming increasingly aware that drugs like Prozac and Sertraline, the most commonly prescribed antidepressants, are having an impact on aquatic life.
The new research has shown that lower than expected concentrations of the drugs in the water will affect the behaviour and biological make-up of these creatures, including changing colour, growing bigger and reproducing more.
In some cases, a lower concentration affected them more than a higher dose.
Dr Alex Ford, a marine biologist from the University of Portsmouth who has led the research published in the journal Aquatic Toxicology, said: "There is a staggering list of prescription drugs passed from humans to wastewater treatment plants and into receiving streams, estuaries, or oceans by direct consumption, metabolism, and excretion or by toilet flushing of old prescriptions.
"Marine invertebrates such as amphipod shrimp become more active and increase their speed of movement while freshwater snails display altered reproduction and some lose their ability to attach to surfaces.
"Some bivalve species, such as zebra mussels, were induced to spawn when exposed to antidepressants.
"In many invertebrates, serotonin controls the release of certain pigments, causing the creature to change colour and recent studies have shown that antidepressants can alter colour changes in cuttlefish. The drugs can also affect growth, feeding and metabolism."
Dr Ford said that just as anti-depressants affect hormones such as serotonin in the human brain, many biological functions within invertebrates are under the control of serotonin.
He added: "What we now know is they can be affected by exceedingly small amounts, as little as one nanogramme per litre - like dropping a few grains of the compound in an Olympic size swimming pool.
"Although concentrations observed in our rivers and estuaries are very small, it's worrying that an increasing number of studies have shown that these incredibly small concentrations can dramatically alter the biology of the organisms they come in contact with."
Dr Ford said that the release of human pharmaceuticals into aquatic ecosystems is an environmental problem we should consider seriously.
He said: "A body of evidence is building that suggests anti-depressants at concentrations currently found in surface, waste and groundwater are sufficient to cause a wide variety of effects. This is despite the fact that reports indicate these types of drugs take up only 4% of the known pharmaceuticals detected in the environment."