Zylkene is a calming supplement that contains a natural ingredient derived from a protein in milk called casein that has clinically proven calming properties to help relax cats and dogs. It is a peptide (protein) molecule, well known to promote the relaxation of new-borns after breastfeeding by delivering ‘calming messages’ to the brain via the body’s natural ‘messenger’ system (scientifically known as Neurotransmitters).