- Exclusion criteria based on sildenafil reference safety information: 1. co-administration with nitric oxide donors or organic nitrates, phosphodiesterase-5 (PDE5) inhibitors, guanylate cyclase stimulators, and most potent of the Cytochrome P450 3A4 (CYP3A4) inhibitors; 2. loss of vision in one eye because of non-arteritic anterior ischemic optic neuropathy (NAION); 3. use of an alpha-blocker; 4. participants with bleeding disorders or active peptic ulceration; 5. known hereditary degenerative retinal disorders such as retinitis pigmentosa; 6. galactose intolerance