There are no medications specifically developed for alleviating separation distress in horses. Some of the medications used to calm horses, such as oral progesterone, l-tryptophan supplementation, or anxiolytics, can be helpful. But in most cases, changing the social environment and management are almost always more useful approaches. If medications are used, your veterinarian should monitor the treatment and health of your horse. One way to help prevent the development of equine separation distress is to systematically expose young horses to separation from pasture mates. In other words, get them used to the comings and goings of herd mates in an organized, gradual way while they are young, and throughout life. If a particular young horse seems stressed by separation, it can be exposed to gradually increasing distances of separation, avoiding any explosive panic events that might set the horse up for future problems.
A certain amount of distress is a normal part of life in animals that have social attachments or home territory, writes Dr. Sue M. McDonnell of the University of Pennsylvania in the November edition of The Horse. A horse usually is upset by separation or isolation from other horses, not from people. It's true that sometimes a horse can be distracted or calmed by the presence of a human, but you almost never see a horse that is initially set off by separation from a human. Common forms of equine separation distress include mare and foal separation and the separation of closely bonded pairs of mares. A gelding turned out with mares may become upset when they are removed from the pasture because he has taken over the natural role of their protector, as surely he would if he were an intact stallion. Depending on the form and severity of the problem, there are some general recommendations to consider. In the case of the protective gelding, one recommendation is to leave a companion behind, one of the mares. Another is to take the horse out of the pasture and reward him with feed. However, in many such situations, a permanent change in management and housing arrangements is the only way to go. For example, many geldings with residual stallion-like behavior would settle down within less than a day or two if they were taken to a place where they were the only horse or where they were with other geldings instead of with mares. In the case of mares that become distressed when separated from a special buddy, most will readily bond to a substitute, or at least they are somewhat calmed by having another companion.