It's from the early seasons. It was primarily focused on in Season 2. Remember Slade? Yeah, that's what he was on in this version. It's essentially the Arrow equivalent of Venom or the SSS. It gives you super strength, makes you really durable and gives you a healing factor. All for the low, low price of driving you insane!
https://arrow.fandom.com/wiki/Mirakuru
And that was just one of Slade's minions.
If Diaz really is doping himself on that stuff now, he might finally be a credible threat.
Unless they remember the cure of course.