Some forms of cysteine are toxic and should be avoided. These include D-cysteine, D-cystine, and 5-methyl cysteine.
NAC may raise levels of homocysteine, an amino acid believed to play a role in the development of heart disease. To be sure, have your Doc check your homocysteine level if you are taking NAC.
Extremely high doses (more than 7 grams) of cysteine should be avoided because it may be toxic to human cells and may even lead to death.