Cystine is an amino acid that is found in digestive enzymes, in the cells of the immune system, and in skeletal and connective tissues, skin, and hair. Hair and skin are 10% to 14% cystine. Amino acids (AAs) are available as single AAs or in AA combinations. They also come as part of multivitamins, proteins, and food supplements. The forms include tablets, liquids, and powders.
By eating enough complete and combined protein sources in your diet, you get all of the amino acids you need.
There are no conditions that increase how much cystine you need.