An erectile dysfunction drug may increase the risk of a common eye condition that can cause blindness.
Those taking the drug tadalafil long-term should have their eye health monitored, scientists warn.
Tadalafil, which is also used to tackle the symptoms of an enlarged prostate, works by relaxing muscles and increasing blood flow.
But the medication is also associated with causing various eye health issues, including conditions affecting the optic nerve and the macula – the part of the retina responsible for sharp detailed vision.
Researchers in Taiwan have discovered there is some suggestion the drug can increase the risk of glaucoma – the leading cause of blindness worldwide, which affects more than a million people in the UK.
Study authors stressed their findings “do not contraindicate tadalafil use” but highlight the importance of monitoring eye health, particularly among people with pre-existing glaucoma or a family history of the condition, as well as patients with high pressure inside the eye.
For the study, published in the British Journal of Ophthalmology, researchers studied data from 73,854 men aged at least 40 with a history of lower urinary tract symptoms.
Half had been prescribed tadalafil or another PDE5i for up to five years, and half were on other drugs.
Men were paired based on lifestyle factors that could influence health, including ethnicity, whether or not they smoked, and other underlying conditions such as diabetes or obesity.
Researchers analysed how likely both groups were to develop glaucoma and to start treatment after five years.
The study found that men on tadalafil were 22 per cent more likely to develop glaucoma and were 32 per cent more likely to develop ocular hypertension.
They were also 33 per cent more likely to develop primary open angle glaucoma – the most common type.
Because this was an observational study, no firm conclusions can be drawn from cause and effect, experts warn.
Symptoms of glaucoma
Glaucoma typically produces no symptoms, leaving most people unaware that they have the condition, according to the NHS.
Because it develops gradually over many years, it is usually identified during routine eye tests.
The condition generally impacts both eyes, though symptoms can be more pronounced in one.
On occasion, glaucoma can manifest suddenly and present acute symptoms.
These include severe eye pain, a red eye, tenderness surrounding the eyes, blurred vision, rainbow-coloured circles around bright lights, feeling sick, vomiting, and a headache.
The NHS advises anyone with concerns about their vision to see their GP or an optician.
