What is a good drug or herbal drug I can give my horse to relax while I clip his head?

I need a drug I can buy at a feed store that works quick so I can clip his head. We have a horse show coming up and I don't have time to work with him with the clippers so I'm needing something to calm him down. I plan on working with him after this show, this is a one time deal with using a drug. Would calf calm or ace work? Help!