L-cysteine is a conditionally essential amino acid, one of only three sulfur-containing amino acids, the others being taurine (which can be produced from L-cysteine) & L-methionine from which L-cysteine can be produced in the body by a multi-step proces Those who supplement with NAC should drink 6 to 8 glasses of water daily in order to prevent cysteine renal stones. Cysteine renal stones are rare but do occur. Take one capsule one to three times daily with or without food, or as recommended by a healthcare practitioner. When taking N-acetyl cysteine it is recommended that two to three times as much vitamin C be taken at the same time.