Diuretics, or 'water pills', make skin cancer seven times more likely, especially in patients who have been taking the drugs for a long time.
The drugs contain a chemical called hydrochlorothiazide, which seems to change the skin, and makes it more sensitive to the sun's UV rays, say researchers at the University of Southern Denmark.
The drugs dramatically increase the risk of one kind of skin cancer, squamous cell carcinoma, which is easily treated, and rarely kills, the researchers say.
It's already known that diuretics make the skin more sensitive, but the researchers were surprised to discover that they can cause skin cancer to develop, and dramatically so.
They reckon that 10 per cent of all cases of squamous cell carcinoma have been the result of the person taking a diuretic that contains hydrochlorothiazide.
Their findings were backed up by Dr Armand Cognetta from Florida State University, who says that his patients, often with hundreds of skin cancers each, have high levels of hydrochlorothiazide in their blood. "The combination of living and residing in sunny Florida while taking hydrochlorothiazide seems to be very serious and even life-threatening for some patients," he said.
Patients who are worried should talk to their doctor about changing to another antihypertensive. Diuretics are the only type that have been linked to skin cancer, the researchers say.