A tablet used to treat sexual problems in hundreds of thousands of UK men could put patients at risk of blindness, a major study has found.

Researchers say patients taking tadalafil – commonly prescribed under the brand name Cialis – may be up to 33 per cent more likely to develop glaucoma. 

Glaucoma, an eye condition where the main nerve connecting the eye to the brain becomes damaged, affects around one million people in the UK.

It is responsible for around 10 per cent of blindness across the nation, and is the leading cause of preventable blindness globally.

Other eye problems have been linked to sildenafil – commonly known as Viagra – which is also used to treat erectile dysfunction.

Experts think the link may with tadalafil may lie with long-term use causing temporary increases to pressure inside the eye, which could gradually damage the optic nerve. 

They also suggested that the drug may contribute to hypoperfusion – reduced blood flow – to the optic nerve. But they stressed that they could not prove those theories.

For the study, scientists from Taiwan and the US analysed 73,854 men over 40, half of whom had been prescribed tadalafil or another, similar erectile dysfunction drug for up to five years.

The results showed that men taking tadalafil were 22 per cent more likely to develop glaucoma than those not taking the drug, and 33 per cent more likely to get the most common type of glaucoma.

They were also 32 per cent more likely to develop high pressure inside the eye, known as ocular hypertension, which can raise the risk of developing the condition.

The researchers stressed that because this was an observational study, it cannot prove that tadalafil causes glaucoma – only that there is a link between the two.

They did, however, say that their findings suggest that long-term use of tadalafil may affect the eyes and should be investigated further.

Tadalafil, which is also used to treat enlarged prostate, also causes side effects including indigestion, muscle aches and pains, a blocked nose and flushed face.

It can also cause headaches, which are a warning sign of glaucoma. 

More than four million men in the UK are affected by erectile dysfunction, and hundreds of thousands are believed to be prescribed tadalafil.

The drugs treat erectile dysfunction by increasing blood flow to the penis, which helps men keep an erection.

For an enlarged prostate, it relaxes the muscles in the prostate and bladder.

The new study's findings come amid warnings that cases of glaucoma are set to rise over the coming decades. 

A separate study earlier this year, also published in the British Journal of Ophthalmology, projected that 1.6 million people will be living with the condition by 2060.

This is a 60 per cent rise from the number of Britons currently living with the condition.

Researchers believe the UK's ageing population will drive the change, as well as the growth of ethnically diverse communities, who are usually at higher risk. 

The NHS says most people do not even realise they have glaucoma.

It is slow to develop and many symptoms such as headaches and nausea may be dismissed as something else.

Other, more obvious symptoms of the condition, include intense eye pain, as well as tender or red eyes.

Seeing rainbow-coloured circles around bright lights or having blurred vision are also tell-tale signs.