Today, there are several varieties of horse shampoo that are marketed for use on human hair. These shampoos claim to leave human hair soft, shiny and luxurious, like the mane of a prize-winning horse. They were originally intended for use on horses, but supposedly when trainers saw the results on their horses, they began using the shampoo on themselves. Since horse shampoo can be harsh, you'll want to use it in a slightly different way than regular shampoo.
Buy a horse shampoo that has been specifically manufactured for human use. These tend to be less harsh than ones made only for horses. Mane 'n Tail, for example, began producing shampoo for humans based on their original horse formula in 1993. Even if you can't find a human formula, however, horse shampoos are perfectly safe for human use. They include ingredients found in many human shampoos, intended to make hair moisturised and shiny.
Dilute the shampoo with water or use only a small amount. Horse shampoo is highly concentrated, which is perfect for their long, thick manes. However, if used in excess, it could be too harsh for your hair and could have some unintended side effects. Since horse shampoo is known for it's moisturising abilities, you could end up with hair that is flat and weighed down if you use too much.
Alternate your use of horse shampoo with another, regular shampoo made for humans, especially if you wash your hair every day. Most people don't need to wash their hair every day, though, so you would do fine using your horse shampoo three or four times a week. If you have particularly dry hair, you can wash your hair a little as once a week with horse shampoo.
Apply conditioner after you use the shampoo. If you have particularly oily hair and don't normally use conditioner, horse shampoo may not be the best option for you since it is known for its moisturising properties. But, if you have dry, difficult to manage hair, horse shampoo would be a great option, as long as your use a soothing conditioner after. Most horse shampoos also have matching conditioners available, so keep your eyes peeled when you go the store.