Viagra (sildenafil citrate) has recently received the Food and Drug Administration (FDA) seal of approval for the treatment of male erectile dysfunction. The recent popularity of this medication, and its availability to the general aviation pilot, warrants a closer look by the aviation medical examiner (AME). With the pilot in mind, the AME should become familiar with certain detrimental side effects of sildenafil.

To date, no written guidelines exist for the use of sildenafil and flying. Pursuant to the Guidelines for Aviation Medical Examiners, all medication use must be reported. However, the "as needed" use of sildenafil may result in confusion for pilots. It is certainly conceivable, given "as needed" dosing and stigmata, that pilots would not report sildenafil use. For the reasons outlined below, it is the author's view that a minimum of 6 hours should pass from "as needed" dosing and flying. Furthermore, the continued (daily) use of Sildenafil is incompatible with safe flight.

The AME should understand the mechanism of action for sildenafil. During sexual stimulation, nitric oxide (NO) is released into the corpus cavernosum. Nitric oxide activates the enzyme guanylate cyclase, thereby increasing the levels of cyclic guanosine monophosphate (cGMP). The cGMP produces smooth muscle relaxation and the inflow of blood into the corpus cavernosum. sildenafil enhances the effect of NO by inhibiting phosphodiesterase Type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum.

When sexual stimulation releases NO, the inhibition of PDE5 by sildenafil increases levels of cGMP in the corpus cavernosum. This results in smooth muscle relaxation, inflow of blood to the corpus cavernosum, and sustained penile erection. Sildenafil at recommended doses has no effect in the absence of sexual stimulation, and has no direct relaxant effect on isolated human corpus cavernosum.

Given the above mechanism of action, potential side effects include:

changes in colour vision
potentiation of nitrate medication

cockpit distraction.
Sildenafil inhibits phosphodiesterae Type 5 (PDE5), however, it also has an affinity and effect on Phosphodiesterase Type 6 (PDE6). phosphodiesterase Type 6 is a retinal enzyme involved in phototransduction. The inhibition of PDE6 results in a mild transient dose related impairment of blue-green color discrimination. Although only 3% of all patients report visual disturbances, this blue-green impairment could be dangerous during Instrument Meteorological Conditions or night flying. The correct identification by the pilot of blue (1) and green (2) is necessary for safe flight. In addition, the use of color video terminal displays has increased in aviation and may present a problem in the color deficient pilot (3).

The AME should also be aware of sildenafil use in the "mile high club" (4). Sildenafil use by a pilot with cardiac disease during sexual intercourse at 5,000 feet, could be deadly. Cardiac disease and nitrate use are risk factors for sudden death during sexual intercourse, not to mention being medically disqualifying. The hypotensive effect of nitrate (Isordil, SLNTG, etc.) is potentiated (5) by sildenafil, consistent with its effect on the NO/cGMP pathway. Recent deaths related to nitrates and sildenafil have made the combination an FDA contraindication.

Finally, the initial dose of sildenafil is 50 mg by mouth 1 hour prior to sexual activity. This dose can be increased to 100 mg, and the drug is rapidly absorbed within 30 to 120 minutes (median 60 minutes). Priapism is not a side effect; however, an early morning flight may be distracting. Full attention to instrument scan and the task at hand may be compromised by the 4-hour half-life of sildenafil. Metabolism of sildenafil by the liver further decreases by 40% at age 65.

For the above reasons, "6 hours from Viagra to throttle" is recommended.