Diazepam is notorious for being irritating IM.  It is recommended to go IV.  Oral absorption is highly variable between species and has not been studied in horses so no dosing guidelines are available for oral.

Topical DMSO on the sore neck may help resolve it. You could put him outside today to see if that helps with his general stiffness.

Eleanor in PA
www.drkellon.com
EC Co-owner
Feb 2001