Juice is the least of what some of the boxers take. I used to workout with a guy who trained fighters in the '80s and he said amphetamine usage was pretty common. Most of what they take has to do with making weight or aggression. He said that the actual true explosive punch-out power was just something that some either had or didn't have. If you had it you could build on it, but no amount of juice or lifting would turn a non-puncher into a true heavy hitter like Tyson.
BTW I have seen Tyson's workout programs, and they are pretty moronic. He pretty much just works chest and biceps. No leg or back work at all. His drug use is probably just as unscientific.
Holyfield on the other hand was trained by Fred Hatfield for a period, and also worked out with Lee Haney. You can bet he knows how to lift and juice.