Enisyl-F contains the essential amino acid L-Lysine. People and animals cannot produce L-Lysine for themselves, so it should ideally be included in their diet. L-Lysine is one of the essential amino-acids and has many useful effects. It can help to increase resistance to infection. Proven effects of Enisyl-F include fewer clinical symptoms, reduced viral excretion, less frequent relapse in carriers of FHV-1.