There's a great documentary on netflix called Icarus. I recommend it to anyone who has some questions or just wants to watch a great documentary on I believe steroids, but could be PEDs, can't remember off the top of my head. They have masking agents, but I'm not gonna pretend to know anything other than what I've read and watched in documentaries.
I think a lot of fighters are on PEDs. Look at all the stupid synthetic pills they sell at gas stations, all the chemicals and shit in them. Now they have a piss test to look for those chemicals they have on their banned substances... They change the chemicals in those things constantly... the DEA rules one of them is illegal, so they change a molecule to something else, that does the same thing, now it's legal.
Don't take this as me saying I know all about chemicals and making those things. I've seen some documentaries on those pills and they go over it in that way.