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Viagra works by increasing blood flow to the penis by inhibiting an enzyme- phosphodiesterase 5 and 6(PDE5 and 6), helping men get an erection. PDE6 is found in retinal cells, and in high doses, it is believed to cause a build-up of a molecule that is toxic to the cells.
Previous research in humans suggests just double the dose will cause problems. In a 2014 study, published in Experimental Eye Research, 50 per cent of healthy men who took 200mg rather than the maximum 100mg experienced temporary visual disturbance.
Australian researchers also said the drug could permanently affect the vision of men with an inherited eye condition, retinitis pigmentosa, after a study on mice. The most common side effects of Viagra are headache, feeling sick, hot flushes and dizziness. Some men have no side effects or only mild ones, according to the NHS.
After drinking a bottle of the liquid, used to treat erectile dysfunction, the man in his mid-50s began to see strange shape.
Viagra can be given on prescription as well as bought over the counter, and previous research has linked an overdose of the drug to visual disturbances.
The man took around ten times the maximum recommended amount of sildenafil, according to the report, drinking 30ml in one go. The bottle contained 750milligrams of the erectile dysfunction drug, compared to the average recommended dose of 50mg.
After complaining of photophobia – a sensitivity to light, the inability to see in low light and the circular shapes in his vision, tests confirmed there was damage. The retina cells are in the back of the eyes and convert light into messages for the brain, which creates a visual perception. They are unable to repair on their own, and degeneration of the cells is normally a cause of blindness.