I've done some searching and all I can find is him handing back his relay medal for the 2000 2 x400 race, because a team mate had admitted it.
https://www.telegraph.co.uk/sport/olympics/2302250/Michael-Johnson-Why-drugs-cheat-shamed-me-into-handing-back-Olympic-relay-gold-medal.html
@Trigaaar...
I didn't realise until today, that he had a massive stroke in September. He is only 50, never smoked, or drank and was at fit as a fiddle.
It just goes to show, you never know.
https://www.bbc.co.uk/sport/athletics/46234803