Hormonal contraceptive use is associated with an increased risk of glaucoma, a study has found, but researchers stress the risk is low and should not put women off taking the medications.

The nested case-control study used US electronic medical records data from between 2008 and 2018 for almost 5 million women aged 15-45 years.

After identifying 2,366 women who developed glaucoma and 9,464 controls, researchers found women who currently used hormonal contraceptives had a more than two-fold increased risk of glaucoma compared with non-users.

The contraceptives used by the cohort studied were intrauterine levonorgestrel, medroxyprogesterone injection, etonogestrel/ethinyl oestradiol (EES) vaginal ring and combined oral contraceptive (COC).

The analysis, published in the British Journal of Clinical Pharmacology, also showed women who had more than four prescriptions for hormonal contraceptives in the last two years had a higher risk of developing glaucoma than those who only took one or two prescriptions.

However, past use of hormonal contraceptives was not linked to an elevated glaucoma risk.

Commenting on the findings, senior author Mahyar Etminan, associate professor of ophthalmology and visual sciences at the University of British Columbia, Canada said: ‘The risk of glaucoma with hormonal contraceptives is low and should not dissuade women from taking these medications.

‘Women on hormonal contraceptives who experience visual changes should have these symptoms examined by an ophthalmologist.’

Two previous epidemiological studies had shown an increase in the risk of glaucoma with use of hormonal contraceptives, the study authors noted.

Several possible mechanisms have been proposed for the association, but it remained unclear whether it could be an effect of oestrogen, progestin or both.

‘The role of progestin suppressing endogenous oestrogen production has been suggested as one mechanism,’ the study authors said.

‘Additionally, the suppression of ovulation by (hormonal contraceptives) also limits total endogenous oestrogen production.

‘Given that oestrogen is hypothesised to exert a protective effect against glaucoma, a reduction in endogenous oestrogen production may potentially confer a greater risk of glaucoma.’