Viagra (sildenafil) is an oral drug used ... An uncommon but potentially serious side effect is priapism in which an erection lasts six hours or more, causing pain and potentially long-term tissue ...

There are no known long-term side effects of tadalafil ... (non-arteritic anterior ischemic optic neuropathy) in a study of sildenafil (Viagra) and optic nerve damage. This condition can develop ...

For most men, sildenafil is a safe and effective treatment for ED when used as prescribed. However, like all drugs, it comes with its own set of potential side effects and interactions.