I know some of you will think I am crazy, but let me tell you what our vet does when he gelds for us. He packs the area with cayenne pepper inside and out. He is from a long line of horse people nad has been doing it for about 25 years now. He says the pepper repels the fly's, breaches the nerve endings and also reduces the swelling.

OH!
He has done over 30 babies for us over the years and it has always worked well, with the new geldings up and walking around a little stiff for several days and then everything goes back to normal. Needless to say I thought he was crazy the first time I saw it.
He also has us apply an external paste 2 days after the gelding is done. The only time we have had to use any drugs for pain was on a 11 year old incorigiable stallion that is now a wonderful gelding. but that was only for a couple of days.