"Our data support the potential benefits of a combination therapy with testosterone gel for men with erectile dysfunction and low testosterone who find sildenafil by itself ineffective," said Dr. Shabsigh of Columbia University. "When assessing erectile dysfunction, doctors and patients should consider using a simple blood test to determine if low testosterone is a contributing factor. If the root cause is low testosterone, sildenafil alone won't fix the problem."

It is estimated that four to five million American men have low testosterone (hypogonadism). Low testosterone may lead to decreased libido, erectile dysfunction, osteoporosis, reduced lean body mass and fatigue.

Sildenafil enhances the effects of nitric oxide, a chemical in the body that allows increased blood flow to the penis during sexual stimulation. "Research suggests nitric oxide may be dependent on testosterone to function properly," said Dr. Shabsigh. "While further studies are needed, this concept may explain why testosterone replacement therapy could help hypogonadal men who do not respond to sildenafil."