N-acetylcysteine (NAC) is a supplement form of of the amino acid cysteine. You get cysteine by eating high-protein foods such as beef, chicken, eggs, and whole grains. Your body can also make some cysteine by combining two other amino acids: serine and methionine. But if you're low on these two amino acids, then you may need to take a cysteine supplement. Your body will convert NAC to cysteine.