Women who take birth control pills for more than three years may have an increased risk of the eye disease glaucoma, according to an investigation by the U.S. National Health and Nutrition Examination Survey.

The group surveyed over 3,400 women over the age of 40. Lead researcher Dr. Shan Lin, professor of clinical ophthalmology at the University of California, San Francisco announced the finding through a news release with the American Academy of Ophthalmology, as WebMD reported. Dr. Lin warns 40-year-old women who have used the pill for more than three years to get their eyes checked for any sign of the disease.

"At this point, women who have taken oral contraceptives for three or more years should be screened for glaucoma and followed closely by an ophthalmologist, especially if they have any other existing risk factors," says Dr. Lin.

It was determined through the survey that women who took birth control pills for more than three years are twice as likely to develop glaucoma. Glaucoma is a disease that causes increased pressure in the fluid in a person's eyes, and can lead to blindness and other symptoms. The disease is estimated to affect 60 million people in the world.

It is unclear what the exact link between Glaucoma and birth control pills is, but there has been a previous study that may hint at what the link may be. Early reports have noted that it appears as though estrogen may be an important part of the puzzle as to how glaucoma develops. Some birth control pills contain estrogen and progestin combined, which could be the link between glaucoma and the pills. However, some birth control pills only contain progestin. While the exact link was not proven, Dr. Lin believes that enough evidence is present to encourage further research into the subject.

Due to the findings of this survey, the scientists in charge of the data believe that doctors should address the possibility of long-term birth control when discussing eye care with patients. Other potential factors for a patient developing glaucoma include previous eye conditions, increased eye pressure, family history of the disease, and the patient's race. 

While this study is a preliminary survey, this link could be a critical find for the millions who both take birth control and the millions who have glaucoma. Perhaps, in time, a more conclusive link can be established between the two factors.