I very much doubt the big DJs are on anything when they’re actually playing, would end up being a total mess.
I’ve heard stories of Lee Foss having to be physically held up during a set due to ‘accidentally’ doing a line of K instead of coke, but your Dixon, Carolas, Väths of this world have to have more professionalism than that - ever so more now that these guys are perilously close to being a viral Facebook video if they were caught a bit worse for wear. Personally I wouldn’t be very impressed if I had paid 50 euro to see someone play a 2 hour set and they couldn’t stay sober for it.