Acepromazine takes about 30 minutes to take effect and will last eight hours or longer. It does not relieve pain, so should not be used to control the horse for painful procedures. Dowling pointed out “it’s important not to ‘trust’ the sedation. If you startle the horse or cause enough pain, he can override the sedation and hurt you.” It causes a significant drop in blood pressure so should not be used if the horse is bleeding or in shock.
Side effects include salivation, sweating, diarrhea, elevated blood glucose, decreased intestinal motility, and the risk of early labour with retained placenta, if given to a pregnant mare in the last trimester of pregnancy. These drugs should not be used with horses who have certain health problems including heart disease, respiratory dysfunction, seizures, or those who are in shock. Further, the heart rate is dramatically slowed, and some horses will faint after begin given a dose.