Humans have been trying to chase away gray hair for millennia. Clay tablets from the Assyrian Empire dated to the 7th Century B.C. mention using the gall of a black ox, cypress oil, licorice, and honey to turn gray hair black.

Ancient Egyptians applied oil cooked with the blood of a black calf, according to the 3,500-year-old Ebers Papyrus. (Presumably, if it worked, we’d still be trying it).

Tech entrepreneur Bryan Johnson, who is spending millions experimenting on himself to slow aging, posted a YouTube video detailing his regimen to reverse hair loss and graying; the video has over 1.5 million views.

Johnson, 46, uses two topical treatments. One contains an herbal extract that, Johnson acknowledges, has colored his hair. But he says something is reversing his grays. When he has looked closely at plucked hairs, he says color has returned to some of them.

What is working? He isn’t sure. Johnson’s routine also includes more than 50 supplements daily and trips to a Honduran island for $25,000 gene-therapy injections.