After decades of research, here it is: the first promising evidence in humans, albeit imperfect and early, that a cocktail of three drugs is enough to reverse the epigenetic clock—a measure of someone’s biological age and health.

The results came as a surprise to even the research team, who originally designed the trial for something a little less dazzling: to look at human growth hormone’s effects on the thymus, the cradle of the body’s immune system that deteriorates with age.

“Maintained immune function is seen in centenarians,” and thymus function is linked to all-cause mortality, explained study author Dr. Gregory Fahy at Intervene Immune, based in Los Angeles, California. “So we were hoping to use a year of growth hormone to maintain thymus function in middle-aged men, right before the tissue’s functions take a nosedive,” he said.