L-Lysine is an essential amino acid that your body can't produce on its own. It is important for the growth and maintenance of bodily tissues, including collagen, cartilage, and skin. L-Lysine supplements can be beneficial for those who don't get enough of this amino acid from their diet or have certain health conditions that require higher levels of L-Lysine. It may also be helpful in supporting immune function and reducing the frequency and severity of cold sores.