Two new drugs, Lilly's Cialis (tadalafil) and Bayer's Vardenafil, may soon muscle their way into the male sexual dysfunction arena.
Like Viagra (sildenafil), all three drugs make it easier for a man with erectile dysfunction (ED) to have an erection after sexual stimulation. But, the new drugs seem to work faster, last longer and have fewer side effects. Here's what we know:
An erection occurs only when genital blood flow is diverted into the central spongy cylinders (corpora cavernosae) of a man's penis, an event that becomes more difficult in men as they age.
Sexual arousal triggers a surge in cyclic GMP, a natural penile blood flow activator that governs the vigor of a man's erection during intercourse. After orgasm— under the influence of an enzyme called phosphoediesterase-5 (PDE-5), cyclic GMP levels fall and a man's erection fades. Viagra, Cialis and Vardenafil are all PDE-5 inhibitors and work to bolster cyclic GMP levels so that a man can have a firmer, more rigid erection during sex. But, there is a difference between the drugs.
The developers of Cialis and Vardenafil have tinkered with the original molecule to come up with the next generation of erection enhancing drugs they believe offer advantages over Viagra.
Ease of Use: Viagra does not work well when taken with a big meal or after alcohol. Cialis works just as well with or without a meal and is effective even with moderate alcohol intake.
Onset of action: Patients taking Viagra are instructed to wait at least an hour before attempting to have sex. Cialis is effective within 15-20 minutes.
Duration of action: After one Viagra dose cyclic GMP levels remain elevated for several hours allowing a man to have sex that evening. The effect of Cialis and possibly Vardenafil seems to be more prolonged. After one dose of Cialis multiple episodes of sexual intercourse are possible.
Europeans have nicknamed Cialis "Le Weekend" and boast that one pill taken with wine and a hearty lunch on Friday is all a man needs to keep him sexually potent on Saturday and Sunday. But, for the mortal man a new pill each day he plans to have sex seems more realistic.
Side effects: Enzymes similar to PDE-5 react to PDE-6 inhibitors. For example, Viagra disrupts Phosphodiesterse 6 (PDE-6) a look— alike color vision enzyme causing blue vision in some Viagra-treated men. Cialis has been engineered to avoid PDE-6 and does not cause blue vision.
The new PDE-5 inhibitors strengthen erections by increasing penile blood flow, but also widen blood vessels elsewhere in the body causing a small percentage of men to experience headache, nasal congestion or upset stomach. Side effects become less common with continued drug use.
Conclusion: Viagra was not just the first pill for ED, but also made it easier for us to talk about sex. With Cialis and Vardenafil even more options will be open to men with ED once they receive FDA approval. Other products are in development and there is significant progress in another long neglected little discussed problem, female sexual dysfunction. Stay tuned.
Richard F. Spark, M.D., is a Senior Attending Physician at Beth Israel Deaconess Medical Center and is an Associate Clinical Professor of Medicine at Harvard Medical School. Dr. Spark is the author of Sexual Health for Men: The Complete Guide. He is also a consultant for Pfizer and Lilly ICOS.