Turning back time: Humans can reverse their biological age. People can roll back the clock with three commercially available drugs.

Age is just a number. But what if you could reduce that number? You actually can, scientists discovered in September this year — if you pay attention to your biological, rather than chronological, age.

During a small, year-long clinical trial, people who took a cocktail of three commercially available drugs managed to turn back the clock on their biology. Participants were given a growth hormone and two diabetes medications. The combination reduced their biological age by 2 1/2 years on average.

Greg Fahy, lead author of the study and chief science officer at Intervene Immune, told Inverse at the time that the findings suggest aging might not be as inevitable as we think.

“One of the lessons that we can draw from the study is that aging is not necessarily something that is beyond our control,†he said. “In fact it seems that aging is largely controlled by biological processes that we may be able to influence.â€

What is ‘biological age?’

While chronological age is the number of years a person has been alive, someone’s biological age is how old the person seems. It’s measured by looking at epigenetic markers, a kind of biological “clock†that traces chemical changes that happen to DNA over time. They’re like “decorations on your DNA,†Fahy said.

An example is methylation — or the addition of methyl groups to DNA. Research published in 2013 suggests there’s a relationship between aging and methylation, making the latter a key metric for epigenetic clocks.

Winding back those clocks may be possible with a trio of already FDA-approved drugs: recombinant human growth hormone (rhGH) and two diabetes medications, dehydroepiandrosterone (DHEA) and metformin.

Using the three drugs, Fahy’s team set out to regenerate the thymus — the immune system’s “master gland.†It takes white blood cells from bone marrow and converts them into T-cells, which fight off diseases like bacteria, viruses, and even cancer. “It’s an essential process for staying alive,†Fahy said.

Around the time humans hit puberty, the thymus begins to lose that vital function. That’s where the growth hormone comes in — rhGH has been shown to reconstitute the thymus in animals. However, the drug can raise insulin levels, which is why the researchers also gave participants diabetes drug metformin, to keep those levels in check.

A hunch led to the third drug, DHEA, being included in the trials. Higher DHEA levels may be the reason young people have higher growth hormone levels without the increased insulin, according to Fahy.

The study comes with a whole host of caveats: It didn’t have a control group, and it included only nine people, all of whom are white men. The researchers want to conduct future studies with larger and more diverse samples.

But the results are encouraging, Fahy said. “It’s so exciting that we’re able to ask the question now, even if we can’t answer it.â€