WHAT IS L-CYSTEINE? L-Cysteine is an amino acid that is only included to a small degree in proteins (2%). Environmental factors can raise the need for L-Cysteine as part of your diet. OUR L-CYSTEINE SUPPLEMENT This individual amino acid supplement provides pure L-Cysteine in capsule form.  EXCIPIENT FREE Our L-Cysteine supplement comes in the form of a vegetable cellulose capsule. This enables us to offer a supplement that does not require excipients, binders or unnatural fillers. A vegetable cellulose capsule is designed specifically to break down naturally within the acid of your stomach, ensuring that the nutrients are absorbed during digestion.