But I'm not more aggressive—a behavior change often tied to testosterone. That's not surprising to Robert Sapolsky, ., a neuroendocrinologist at Stanford University and a leading researcher on stress and behavior. "It's really not the case that testosterone 'causes' aggressive behavior," he says. "Instead, it makes the brain more sensitive to social cues that trigger aggression. And in support of that, a guy's testosterone level isn't a very good predictor of how likely he is to be aggressive."
Meta-analyses of placebo-controlled trials suggest that testosterone therapy in physiological doses is significantly associated with increased haematocrit, reduced high-density lipoprotein cholesterol and prostatic symptoms. 29 , 30 If prostate cancer has been excluded, there appears to be no increased risk of induction by testosterone therapy. There is inconsistent evidence regarding the risk of cardiovascular events. 29-31 A recent meta-analysis suggested increased cardiovascular risk and reported publication biases. 32 Long-term safety data are lacking, but recent reports more strongly suggest an increased risk of cardiovascular events in older men. 3 , 4 This has prompted the Endocrine Society to issue a warning statement. 5 The results and safety of long-term prospective controlled trials of testosterone therapy are awaited.
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.