How Viagra Works

Viagra Competitors

Viagra is a hugely successful drug, so other drug companies of course want a piece of the action. They developed different chemicals to block the PDE5 enzyme and created two new drugs: Cialis (tadalafil) and Levitra (vardenafil). (There is another drug, Staxyn, with the same active ingredient as Levitra that has recently come onto the market. It's an orally disintegrating tablet that isn't interchangeable with Levitra, but chemically is much the same.)

Because Cialis and Levitra block the PDE5 enzyme, they work exactly the same way as Viagra. They help men who have trouble maintaining an erection because of blood flow problems, and they only work when the man is sexually aroused.


Because they block PDE5 with different chemicals, however, there are some differences between the three drugs. For example:

  • Cialis causes muscle aches and back pain in some patients, a side effect that isn't associated with Levitra or Viagra.
  • Viagra lasts about four hours in the bloodstream, while Levitra lingers for five. Cialis stays in the bloodstream much longer (it has a 17.5-hour half life) and can therefore be effective for more than a day -- for this reason, Cialis is expected to outsell Viagra soon [source: Wilson]. Its nickname? "The Weekender."
  • Cialis works faster than Levitra or Viagra -- in about 15 minutes.
  • Fatty foods seem to affect how Viagra and Levitra work in the body but don't affect Cialis.

There are always new drugs in development. Here are a few future possibilities:

  • Uprima (apomorphine) is available in Europe, but not the U.S. It works quickly and may affect sexual interest, but it also causes nausea and, sometimes, syncope (fainting). It acts on the central nervous system. There is also a nasal spray version in development. (Interestingly, it used to be used in aversion therapy to try to "convert" gay men to heterosexuality -- the nausea side effect came in handy.)
  • Topiglan (alprostadil) would be a topical version of the drug that is even now used in injection or suppository form to fight ED. This might be very helpful for men whose other medications make it impossible for them to take oral ED drugs safely.
  • Avanafil, another PDE5 inhibitor that may work faster and have fewer side effects than what's currently on the market. It is in phase III clinical trials.

Want to learn more about erectile dysfunction and sexual conditions? There are lots more links below.

Related Articles


  • Aviation Medicine Advisory Service. "Medication Class: Male Impotence Medications." (May 9, 2011)
  • Boston University. "Better Erections with Injections." Boston University School of Medicine. May 17, 2004. (May 15, 2011)
  • Downs, Martin F. "5 Things You Didn't Know About Your Penis." WebMD. March 17, 2010. (May 10, 2011)
  • Giuliano, F. and J. Allard. "Apomorphine SL (Uprima): preclinical and clinical experiences learned from the first central nervous system-acting ED drug." Abstract. International Journal of Impotence Research. February 2002. (May 8, 2011)
  • Goldstein, Irwin M.D. "The Current and Emerging Medical Therapies for Male Erectile Dysfunction." Boston University. March 30, 2003. (May 15, 2011)
  • Heaton, J.P. "Characterising the benefit of apomorphine SL (Uprima) as an optimised treatment for representative populations with erectile dysfunction." Abstract. International Journal of Impotence Research. August 2001. (May 8, 2011)
  • Hitt, Emma. "FDA Advisory Committee Votes Against Flibanserin for Hypoactive Sexual Disorder." Medscape Medical News. June 21, 2010. (May 9, 2011)
  • Jonas, U. "The history of erectile dysfunction management." International Journal of Impotence Research. 2001. (May 8, 2011)
  • Lakatta, Ed G., M.D. "Nitric Oxide and Vascular Health -- Article XV (Part 2 of 4)." April 7, 2009. (May 16, 2011)
  • Marks, Leonard S., M.D. "Viagra vs. the new PDE-5 Inhibitors: Levitra and Cialis." Urological Sciences Research Foundation. Fourth quarter 2003. (May 15, 2011)
  • McCullough, Andrew R. "An Update on the PDE-5 Inhibitors." Journal of Andrology. November-December 2003. (May 8, 2011)
  • Melnick, Meredith. "Could a New 'Viagra Condom' Encourage Safer Sex?" Time Healthland. May 9, 2011. (May 9, 2011)
  • Nobel Prize. "Physiology or Medicine for 1998 -- Press Release." Oct. 12, 1998. (May 15, 2011)
  • Roehrborn, Claus G. M.D. "Lower Urinary Tract Symptoms, Benign Prostatic Hyperplasia, Erectile Dysfunction, and Phosphodiesterase-5 Inhibitors­." MedReviews: Reviews in Urology. Summer 2004. (May 15, 2011)
  • SchoolScience. "The Viagra story." (May 15, 2011)
  • Soni, Sangha. "Viagra is Huge in Mexico -- What's up with That?" Fox News Latino. May 5, 2011. (May 9, 2011)
  • WebMD. "Erectile Dysfunction: Cialis, Levitra, Staxyn and Viagra to Treat ED." July 31, 2010. (May 9, 2011)
  • WebMD. "Erectile Dysfunction Guide." (May 7, 2011)
  • Wilson, Duff. "As Generics Near, Makers tweak Erectile Drugs." The New York Times. April 13, 2011. (May 10, 2011)