Vision phototransduction

Adverse effects are short-lived, dose-related, and comprise headache, flushing, nasal congestion and dyspepsia. High doses can inhibit PDE6 which is needed for phototransduction in the retina, and some patients report transient colour vision disturbance. (The more recently developed PDE5 inhibitors, cialis and vardenafil, appear less likely to cause visual upset.) Priapism has been reported. 

The process of vision is a sequence of changes in phototransduction of photosensitive pigments [rods and cones]. These events may be summerise as follows ... 

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