Most horse foods and forage in the Uk are high in iron so supplementing is usually not required and too much iron will prevent the absorbtion of other minerals such as copper and magnesium (which are vital for good health)
What is your need for it? My mare has been on it for about a month now because she was severly anemic. It's not really ment as a normal supplement to regularly give your horse. Once my one container of it is gone my mare will be off of it. It works wonders if your horse actually needs it.