A nutrient in the vitamin B complex that the body needs in small amounts to function and stay healthy. Vitamin H helps some enzymes break down substances in the body for energy and helps tissues develop. It is found in yeast, whole milk, egg yolks, and organ meats. Vitamin H is water-soluble (can dissolve in water) and must be taken in every day. Not enough vitamin H can cause skin, nerve, and eye disorders. Vitamin H is present in larger amounts in some cancer tissue than in normal tissue. Attaching vitamin H to substances used to treat some types of cancer helps them find cancer cells. Also called biotin.