Is there any medical reason why a person would lose their eyesight after taking and ED drug and how common is that?

See answer below. While vision loss is listed among the most serious adevrse events of therapies used for erectile dysfunction such as Cialis for example, it is not a common side effect.
Remote possibilty. There is a condition called anterior ischemic optic neuropathy (aion). It is generally seen in older patients with pre-existing vascular disease secondary to hypertension, diabetes, and/or hypercholesterolemia. Pde5 inhibitors used to treat ed have been associated with aion. So any symptoms of visual darkening or loss should prompt discontinuation and seeing dr.