Firstly, Trihist is an antihistamine that is used for some horses with allergies. Like people with allergies, a horse's response to antihistamines can be very satisfactory or not at all satisfactory.
The signs that your mare is demonstrating seem to be more consistent with what is termed a "head shaker." These horses often respond to a stimulus by shaking or tossing their head excessively. Some individuals seem to be stimulated by bright light (the sun) and will do well in a covered arena but may be unrideable in the open on a bright, sunny day. Others are affected more on a very breezy day. It is often very hard to determine the cause of the condition, and depending upon the cause, treatment may be difficult.
To try to get to the origin of your mare's problem, we recommend working with a veterinarian who is familiar with this condition and would keep a very detailed journal of her behavior and the conditions that she is in at the time; for example, sunny, windy, warm, etc.; tack used for riding; whether she was O.K. to start and then got worse, etc. Also, if you have not already done so, make sure that her mouth and teeth are O.K. and that her upper airway and guttural pouches are normal.
