Using testosterone along with Sildenafil citrate (Viagra) does not increase chances of erection in men who have been diagnosed with erectile dysfunction, a new study found.

As slidenafil doesn't always restore erectile function in men who have erectile dysfunction, doctors prescribe testosterone to some men.

The study included 140 men between ages 40 and 70 with erectile dysfunction. Participants were divided into two groups; first received daily transdermal gel containing 10g of testosterone while the other group got placebo for 14 weeks.

"We had expected to see some added benefit from testosterone. But that's not what we observed," said lead researcher Dr. Matthew Spitzer, an endocrinology fellow at Boston University School of Medicine, HealthDay reported.

Researchers found that testosterone was no better than placebo in treating erectile dysfunction in men who were using Viagra.

"While low testosterone and [erectile dysfunction] ED can be related, they are separate issues, and I treat them as such. I don't expect that treating low testosterone will improve ED, but it will likely improve libido and interest in sex," said Michael Eisenberg, MD, an assistant professor of urology at Stanford University School of Medicine, WebMD reports.

Testosterone gels are associated with acne, prostate enlargement (which can cause urinary problems) and an increased risk of prostate cancer.

Erectile dysfunction affects as many as 30 million men in the U.S., according to National Kidney and Urologic Diseases Information Clearinghouse.

The study is published in the journal Annals of Internal Medicine.