L-lysine is an essential amino acid (meaning your body cannot manufacture it on its own) that acts as a natural stress fighter. It’s also highly effective at promoting a healthy immune response. L-lysine is required for normal growth and development in infants and for maintaining nitrogen balance in adults. Good sources of L-lysine include meat, fish, legumes, and dairy products. Swanson sells several high-quality, and highly-rated L-lysine supplements from a variety of brands to fit your needs. Check out some of the products and their reviews below.