Answer: Viagra, or sildenafil, acts to inhibit phosphodiesterase 6 (PDE6). Inhibition of this enzyme causes the rod cells in the eyes to become more sensitive, resulting in cyanopsia, or blue tinted vision.
Originally formulated as a medication to lower blood pressure, sildenafil or Viagra, is a chemical that is known to inhibit the function of the enzyme phosphodiesterase (PDE). Through inhibition of PDE, it can improve sexual performance by dilating blood vessels in the penis, allowing increased blood flow. It belongs to a class of drugs called phosphodiesterase inhibitors, or PDEis. They are being tested widely in the clinical setting for the treatment of various conditions.
Viagra’s main action on the body that improves erectile dysfunction is to inhibit the action of PDE5, resulting in blood vessel dilation. However, the action of the drug is not specific to PDE5, as PDE6 is also affected. PDE6 has a major influence in the eyes, particularly in the rod type photoreceptor cells. These photoreceptors are tuned to maximally activate at around 500 nm wavelength, which corresponds roughly to blue-green. When PDE6 is inhibited in the eyes, these rod type photoreceptor cells become sensitized, meaning they respond more robustly to stimuli. Therefore, your vision may be tinted blue as a result.
The enzyme phosphodiesterase functions in the body by degrading cyclic signaling molecules such as cAMP and cGMP. The enzyme breaks apart the cyclic phosphodiester bonds. PDEs themselves come in a variety of subclasses, called PDE1 through PDE11.