This is one of those questions that, ASSUMING YOU AREN'T bullshitting, indicates that you do not have the knowledge to even be making the attempt. The drugs that will make a horse drop are DANGEROUS when you don't know what you're doing with them. Overdose = dead horse. Underdose can (and I've seen it happen often enough to be scary) do EXACTLY the opposite - horse goes into total freakout mode, rather than being sedated. Can equal dead YOU.
Long and short of it, if you (actually, as opposed to wanting a shortcut to get a horse dick hanging out that you can play with) need to drug him to clean him, the only SAFE way is to get the vet out and pay the price of your failure to teach him to drop on command.