N-Acetyl-Cysteine(NAC) is the precurser of Glutathione the main cell protecter against cancer. It coats the cell with a protective shield against cancer,however once cancer cells are established and spreading NAC will also protect the cancer cells in the same manner against any chemo or natural supplement attempts to kill off the cancer cells.

Supplements such as green tea,curcumin,berberine,ginseng,selenium,melatonin and vitamin D3 have shown to be neutralised in killing off cancer cells in in vitro cell lines once NAC is applied.

Whey protein also raises cysteine levels and will cause the same problem in protecting the cancer against apostosis.

https://denvernaturopathic.com/NAC2011.htm

That is extremely good info. I knew that vit e and seleniuma nd vit C should not be given with a few days of cancer treatment because they also protect cancer cells.