A widely prescribed erectile dysfunction tablet could increase the risk of glaucoma and potentially threaten patients' eyesight, new research suggests. Glaucoma affects around one million people in the UK and is responsible for around one in 10 cases of blindness. The results showed men taking tadalafil were 22 per cent more likely to develop glaucoma overall and 33 per cent more likely to develop primary open-angle glaucoma, the most common form of the disease. The researchers emphasised that the observational study does not prove tadalafil causes glaucoma, only that an association exists. However, they concluded the findings indicate the long-term effects of the drug on eye health deserve further investigation.