I know this doesn't have any value but I've read a study somewhere that determined similar results.
Invalid Link Removed - "While N-Acetyl Cysteine is a more stable compound than taking oral cysteine, as it is metabolized, some NAC may be oxidized and become insoluble. This may form kidney stones. It is therefore recommended that persons taking NAC take at least as much vitamin C as NAC. Diabetics should consult their physician before using N-Acetyl-Cysteine, since it may have an insulin-blocking effect."