context: Testosterone isn't the only fuel for a man's sex drive and performance. But low testosterone can reduce your ability to have satisfying sex. Lack of sex drive and erectile dysfunction are sexual problems that can result from low testosterone. If low testosterone is the cause, treating it can help. Researchers haven't unraveled the mystery of just how testosterone increases libido. It's normal for a man's sex drive to slowly decline from its peak in his teens and 20s, but libido varies widely between men. What one man might consider a low sex drive, another might not. Also, sex drive changes within each man over time and is affected by stress, sleep, and opportunities for sex. For these reasons, defining a "normal" sex drive is next to impossible. Usually, the man himself identifies a lack of sex drive as a problem. Other times, his partner may consider it to be an issue. Low testosterone symptoms don't always include feeling like you have no sex drive. Some men maintain sexual desire at relatively low testosterone levels. For other men, libido may lag even with normal testosterone levels. Low testosterone is one of the possible causes of low libido, however. If testosterone is lowered far enough, virtually all men will experience some decline in sex drive. In a large study of men in Massachusetts, about 11% overall said they had a lack of sex drive. The researchers then tested all the men's testosterone levels. About 28% of men with low testosterone had low libido. These men were relatively young, with an average age of 47; older men might have worse sexual symptoms. Low testosterone is only one of the causes of low libido. Stress, sleep deprivation, depression, and chronic medical illnesses can also sap a man's sex drive. Surprisingly, low testosterone by itself rarely causes erectile dysfunction, or ED. Low testosterone alone -- with no other health problems -- accounts for a small minority of men with erectile dysfunction. Erection problems are usually caused by atherosclerosis -- hardening of the arteries. If damaged, the tiny blood vessels supplying the penis can no longer dilate to bring in the strong flow needed for a firm erection. Diabetes, high blood pressure, smoking, and high cholesterol are the three main causes of atherosclerosis and erectile dysfunction. At the same time, low testosterone is a frequent accomplice to atherosclerosis in creating erectile dysfunction. In studies, as many as one in three men mentioning ED to their doctor have low testosterone. Experts believe that in men with other factors causing erectile dysfunction, low testosterone can strongly contribute, making a difficult situation even worse. Strengthening the connection, low testosterone is linked in some way with many of the conditions that lead to erectile dysfunction: Metabolic syndrome Obesity Endothelial dysfunction Diabetes Although low testosterone isn't known to cause them, the associations between other medical conditions and low testosterone can be significant. Testosterone therapy improves sex drive and satisfaction with sex in many men. However, the long-term risks and benefits of testosterone replacement are unknown. Research trials on testosterone replacement in men are ongoing, although results are years away.