Recent news reports have been full of stories about the new drug Viagra, which can relieve some patients' problems with impotence. Evidently the side effects include abnormalities of vision. The linkage between the effects and side effects are easy to understand from a biochemical perspective.

The engorgement of blood which occurs during sexual excitation is produced in response to the neurotransmitter Nitric Oxide, or NO. The second messenger for NO is cyclic GMP, analogous to cAMP. Thus, a drug to enhance sexual performance ought to either provide Nitric Oxide to the tissue or prevent removal of cGMP.

Viagra, or Sildenafil, is an inhibitor of PDE-V, a phosphodiesterase which breaks down cGMP to regular GMP. This produces the effect of the drug. The "side effect" of altered vision relates to the fact that vision is also mediated by cGMP. This is described in Stryer's "Biochemistry" 4th edition chapter 13, see pages 332-339, and in Garrett and Grisham's paperback "Molecular Aspects of Cell Biology" pages 1244-1250.

  1. Sildenafil
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  3. PharmInfoNet
  4. Urology Times
  5. The Nitric Oxide Home Page