) Tri-Hist Granules are used as an antihistamine and decongestant for horses. Tri-Hist Granules contain pyrilamine maleate and pseudophedrine HCl. Each oz of Tri-Hist Granules contains 600 mg pyrilamine maleate and 600 mg pseudophedrine HCl. The recommended dosage of Tri-Hist Granules is 1/2 oz (1 level Tbsp) per 1000 lbs body weight. Can be mixed with feed and repeated at 12 hour intervals, if needed. Pyrilamine maleate and pseudophedrine HCl.