Women who take the contraceptive pill for more than three years are at twice the risk of glaucoma, a common eye disease that can cause blindness if left untreated, US researchers claim.

Doctors behind the study urged women at risk to have regular checks for early signs of the disease, which can usually be treated with eye drops if it is caught in time.

Glaucoma is caused by a buildup of fluid pressure in the eye which damages the sensitive optic nerve at the back of the retina. The most common form affects around 480,000 people in England, according to the NHS.

Dr Shan Lin, an ophthalmologist at the University of San Francisco, said long-term use of the contraceptive pill appeared to raise the lifetime risk of glaucoma to around 5%, up from 2.5% in the general population. He presented details of the study at the American Academy of Ophthalmology meeting in New Orleans on Monday.

Lin drew on medical and lifestyle information given by 3,406 women aged 40 or older in the US national health and nutrition examination survey. He found that women who took any sort of oral contraceptive pill for longer than three years were twice as likely to report glaucoma than other women.

"If you are a doctor you want to be able to counsel your patients and at least be aware that they may be at higher risk for developing glaucoma," Lin said. "If you are taking the pill, and particularly if you have a family risk of glaucoma, you may want to have regular screening," he added. Apart from a family history of the disease, other risk factors for glaucoma include short-sightedness, black African ancestry, raised pressure in the eye, and diabetes.

In the study, 231 women who had spent more than three years on the pill had been diagnosed with glaucoma, compared to an expected 115 for their age range.

"Catching glaucoma early by seeing an ophthalmologist is very important. If we can catch it early and prevent it we can prevent almost everybody from developing blindess from this condition," Lin said.

Scientists are not sure whether the pill can cause glaucoma directly, and the latest study did not distinguish between different types of pill. Lin speculates that the pill might cause damage to retinal ganglion cells by flattening out levels of oestrogen in the woman's bloodstream.

In 2011, a larger study of nearly 80,000 US nurses found that those who took the contraceptive pill for five or more years had only a 25% increased risk of glaucoma, meaning their chances of developing the disorder over a lifetime rose from around 2.5% to just over 3%.

"These new findings are interesting but do not confirm a link between long term use of oral contraceptives and a raised risk of glaucoma," said Clara Eaglen at the Royal National Institute of Blind People. "Further research is needed to examine whether there is a definite association."