PDE5A and erectile dysfunction: Due to the similarity in amino acid sequence and catalytic structural domains between PDE6 and PDE5 (Pissarnitski, 2006), when patients with erectile dysfunction (ED) take PDE5 inhibitors, cross-reactions at the catalytic site of PDE6 can lead to visual disturbances such as functional blindness, cyanopsia, blurred vision, and increased photosensitivity (Foresta et al., 2008; Kerr and Danesh-Meyer, 2009).