Headshaking in horses is a medical condition recognized by veterinarians, with ongoing research being performed to best diagnose and treat this syndrome.
While all horses shake their heads in response to flies and other insects, horses with headshaking syndrome display a repetitive, involuntary up-and-down, vertical motion that is more severe and predictable than normal headshaking
Since a main cause of headshaking is the hypersensitization of the trigeminal nerve, anything to reduce or calm the firing of this nerve may be beneficial. If photic headshaking is thought to be a problem, using a 90% UV blocking mask and keeping the horse in a screened or darker stall during the day, with turnout at night can be tried.
A lighter mask or nose net while riding, and exercising an affected horse in an indoor arena may help to reduce nerve stimulation.
Magnesium is an electrolyte that can have a calming effect and may help reduce hypersensitivity of the trigeminal nerve. Supplementation with higher than normal doses may be advantageous but should only be done under veterinary care and monitoring. Magnesium absorption can be enhanced when supplemented in combination with boron.