A number of people have reported some of their gray hair returning to its original color after taking NR for months or years. I have even seen a "smoking gun" hair that was brown at the root, but gray at the tip. We have also seen control mice in anti-aging studies that seem to have less gray hair.

Interestingly, though, where the effect is reported, it appears to be only partial -- some hairs, but not all, return to their normal color.

This is, of course, a difficult study in humans, and no one appears to be attempting it, so we may not know for a long time.