UK drinking water supplies are to be tested for possible contamination by widely-prescribed and potentially dangerous prescription drugs, particularly cytotoxic anti-cancer medicines which can be excreted unaltered into the sewerage system.

The government has ordered a pilot testing project to begin early next year at four drinking water intake points along a major river - which is widely expected to be the Thames, the Independent newspaper has reported. The sites, and the prescription drugs which the tests will look for, are to be selected over the next few weeks. Testing will continue for at least a year, and the scientists will also test water after it has been purified to drinking standard.

European Union (EU) rules state that drinking water must be monitored for 48 different contaminants, but these do not include pharmaceutical active ingredients. Particular concerns have been raised about the presence of cytotoxic cancer drugs - prescriptions for which are increasing about 10% a year - as, with more and more patients taking them at home, they can pass unaltered into the domestic sewerage system. These drugs also dissolve easily in water, can survive conventional purification methods and could potentially have additive effects when several different treatments are absorbed together. Even if they do not prove harmful to adults, they could affect infants in the womb, it is believed.

The testing pilot has been commissioned by the Department for Environment, Food and Rural Affairs (Defra) and the Drinking Water Inspectorate (DWI), and will be carried out by a consortium of laboratories led by Defra’s Central Science Laboratory (CSL). Officials have been quick to reassure the public – CSL environmental chemist Alistair Boxall, who will oversee the tests, said he found it hard to believe pharmaceuticals could have any effect because of the amount of water which would have to be drunk to get close to a viable dose, and a Defra spokeswoman pointed out that the safety of drinking water is testing routinely by the DWI and the water companies. The new study would provide new information to support a previous risk assessment which had concluded that there was no significant risk to human health, she added.