Yes, we’ve read about how testosterone can increase strength, sex drive and confidence, but a 2015 study from researchers at Harvard Medical School suggests that for most men, that is, those whose testosterone is within the normal range, testosterone does almost nothing. The New York Times reports on the research, which found that the biggest benefits of testosterone supplements, in terms of energy and sex drive, were enjoyed by men with clinically low testosterone. What’s low? The Times says the normal range is between 300 to 500 nanograms per deciliter of blood. Those who got the biggest bang for their buck had levels below 230. According to the Times, testosterone levels start to drop around age 30, and they decrease on average about 1 percent a year. A few small studies show that about 5 percent of men between the ages of 50 and 59 have low levels of testosterone along with symptoms like loss of libido and sluggishness.