Viagra has been linked to a doubling in the risks of eye conditions that can cause blindness, US research suggests.
The study of more than 200,000 men found that those taking drugs for erectile dysfunction were twice as likely to develop rare conditions which can result in loss of vision.
However, scientists said it was not clear whether the drugs had caused the problems - or whether underlying health conditions which increase the risk of impotence could be to blame.
The study by the University of British Columbia (UBC), published in Jama Opthalmology, analysed the health insurance claim records of 213,000 men who were regular users of common erectile medications such as Viagra, Cialis, Levitra and Stendra.
Since 2018, Viagra has been available over the counter in Britain, with surveys suggesting around one in six men have taken the drug, or other medication for erectile dysfunction.
The new research found that those taking such pills had a 2.5 fold rise in the risks of serious retinal detachment, and twice the chance of ischemic optic neuropathy, which compromises blood supply to the optic nerve. The risk of retinal vascular occlusion - a blood clot in the veins or arteries of the retina - rose by 44 per cent.
All such conditions can cause sight loss, but are extremely rare, experts stressed, with around one in 1000 people suffering retinal detachments.
Dr Mahyar Etminan, a UBC associate professor in the department of ophthalmology and visual sciences, said: "These medications address erectile dysfunction by improving blood flow, but we know that they can also hinder blood flow in other parts of the body.
"So although our study doesn't prove cause-and-effect, there is a mechanism by which these medications could conceivably lead to these problems. The totality of the evidence points toward a strong link.
"These are rare conditions, and the risk of developing one remains very low for any individual user."
But he said the high numbers of prescriptions being dispensed for erectile dysfunction meant that "a significant number of people could be impacted".
"Regular users of these drugs who find any changes in their vision should take it seriously and seek medical attention."
The study attempted to take account of other conditions such as high blood pressure, diabetes and heart disease, which could also increase such risks.
But experts said some underlying conditions may have been missed, and said that only a randomised controlled study could show if the increased risk was caused by the medication.
Obesity and diabetes also affect blood vessels
Robert MacLaren, a professor of ophthalmology at the University of Oxford, said: "Phosphodiesterase inhibitors [e.g. Viagra] are an effective treatment for men with erectile dysfunction. The research in this paper implies that there might be a slightly increased risk of blood vessel problems in the eye in regular users of this medication.
"However, it must be remembered that the conditions that lead to erectile dysfunction, such as obesity, diabetes, and high blood pressure also affect the ocular blood vessels. Hence this may simply be an association of two conditions, rather than one causing the other."
He said the relative risks appeared to be "very low" saying those taking the medication should not have cause for concern.
Dr Dipender Gill, an NIHR clinical lecturer in clinical pharmacology and therapeutics at St George's, University of London, said: "While the authors are correct in their assertion that the findings warrant further pharmacovigilance regarding such possible harmful effects of phosphodiesterase 5 inhibitors, we can't say from this study alone that the drug class caused these adverse events."
Linda Sharples, a professor of medical statistics at the London School of Hygiene & Tropical Medicine, said: "The cohort study tells us that these vision impairments are fairly rare, occurring in 1.55 patients per thousand in this population."
She said the extra risk would mean an additional 1.3 patients per thousand people who are treated with these drugs being diagnosed with visual impairment.
"Because there is no clear biological mechanism for the relationships, we cannot be sure that all confounding has been adjusted for, so that a causal relationship cannot be claimed."
But she said the dose response shown in the research, and the consistency of the findings, meant further studies might be worthwhile.
A spokesperson for UK safety regulators the Medicines and Healthcare Products Regulatory Agency said: "Sildenafil, tadalafil, vardenafil and avanafil are important medicines for the management of erectile dysfunction, and use of these products is known to be associated with a risk of visual problems.
"The study published in JAMA Ophthalmology confirms the known risk of certain visual problems which may rarely occur in regular users of these medicines.
"The product information for these medicines advises that if any sudden visual defect occurs, patients should stop taking the product and consult a doctor immediately.
"Patients can continue to use these products, but should discuss any concerns about their treatment with their doctor or pharmacist."
Viatris, manufacturers of Viagra, did not respond to requests for comment.