Horsetail has not been proven to grow hair, but the herb is thought to replenish the minerals in the diet (such as selenium) which are known to promote healthy hair growth.  However, the use of nutritional supplements is not known to be effective for everyone, in fact, there is limited research available on dietary supplementation and loss of hair.  Despite the lack of research data, many hair loss products have “active ingredients’’ that includes selenium. It’s important to note that selenium toxicity is well documented and one side-effect of taking too much selenium is hair loss.