Ever since Pfizer Inc.'s Viagra oral sildenafil took the market for erectile dysfunction therapies by storm, competitors have been scrambling to reposition their products or reiterate why they believe their positioning is correct.

Viagra's first victim, Vivus Inc., saw U.S. sales of its Muse alprostadil urethral suppository decline by nearly 50 percent in the second quarter of this year. But in the face of Viagra's dominance, Senetek plc (SNTKY) is confident that its Invicorp injectable combination of phentolamine mesylate and