Men who've, ahem, fallen and can't get up - even with the help of Viagra® - may want to get their testosterone levels checked. A recent study shows that in cases where men experiencing erectile difficulties fail to respond to medication, low levels of the male sex hormone may play a role.

Sildenafil, which is sold under the brand name Viagra®, helps men to achieve and maintain erections by increasing the blood flow to the penis. While circulation problems are the number one cause of erectile difficulties, low testosterone levels can lower your sex drive and result in problems getting erections or result in erections that are not as strong as usual. What's more, if you do have low testosterone, sildenafil alone might not be enough to get the results you want.

A study published in the journal BJU International assessed the effectiveness of sildenafil in 162 men who were 60 years of age or older, and found the medication to be effective at improving erections in only 47% of cases. Among the men for whom sildenafil didn't work, low testosterone was found to be one of the strongest predictors of the medication's failure. Particularly poor erectile function prior to using sildenafil and smoking were also associated with the medication's failure to work.

The study is the latest to suggest that, in cases where men have difficulties achieving erections, a wider-scale problem may be to blame. The authors of a study published in the Journal of Urology recommended that men who don't respond to sildenafil be tested for low testosterone and receive testosterone replacement if necessary.

They made the recommendation after comparing the effects of testosterone replacement used in conjunction with sildenafil versus a combination of a placebo and sildenafil on 75 men who had low testosterone levels and who previously did not respond to sildenafil alone. After four weeks, there was significant improvement in the erections of men in the testosterone replacement group in comparison to the placebo group.

Low testosterone can also affect more than your sex life. Other symptoms include low energy levels, a decrease in strength and muscle mass, increased body fat, irritability, and depression.

While this condition can be caused by injury or a number of medical conditions, testosterone levels also tend to decrease naturally as men age - by about 10% per decade after the age of 30. But just because a decrease in testosterone is a normal part of the aging process, it doesn't mean you're stuck with the symptoms associated with low testosterone.

Low testosterone can easily be diagnosed with a blood test, and there are a number of treatment options available to boost levels of this hormone and reduce your symptoms. These include a patch, which is worn around-the-clock to mimic the natural daily fluctuations of testosterone in the bloodstream, as well as testosterone pills, injections, and gel.