It's hard to say but just remember that you get paid the most when you're on top. If you could take something that would give you an advantage without getting caught, would you?
I'm going to say less than 20%. Even that is a really high number when compared to the level of positive tests when MLB and the NFL instituted testing. Can I see 1-2 guys per weight division on something? Yes.
Nowadays everyone is pointing the finger, but I think the most likely to abuse because of the lack of scrutiny are guys who are almost there at the bottom of the top 10 contender list who don't seem to be able to land recognition, fly under the radar, and are maybe just so close to greatness but can't quite get it. I liken it to Clottey and Chop Chop Corley, who are good enough to just lose to everybody. That has to be frustrating: you are not a superstar and don't make much money, but you are almost on the level of those A guys, so it must be a big temptation to just get a payday or two with a great performance with a little extra edge. I think the guys on the very top, if they care about their legacy, are too scared of damaging their reputation (Pac and Floyd, to beat the dead horse - anyone who is a hero to their country might think twice about being outed as someone who cheated to get there). I think 20% sounds right, but some on here would have us believe 100% are on it, and I just don't think so.
I say above 20%. In this day and age, i would guess that there are so many substances that one can take that won't be detected in urine tests. And when that drugs gets detected, they will move on to another, newer drug that won't be detected.
Some of the best fighters of this era got caught using banned substances. RJJ, Toney, Mosley, Holyfield, etc.