Because of the nature of cancer cells (greatly reduced ability to self-regulate) providing additional cysteine has a paradoxical effect - the precursors that raise glutathione in normal cells trigger the opposite reaction in cancerous cells.
When glutathione production is overstimulated in cancerous cells, they shut down glutathione production in a process called "negative feedback inhibition", making them more susceptible to destruction. Meanwhile, normal tissue uses the precursors to make glutathione, leaving healthy cells with better defenses which is very important while undergoing treatments.
Russo A, DeGraff W, Friedman N, et al. Selective modulation of glutathione levels in human normal versus tumor cells and subsequent differential response to chemotherapy drugs. Cancer Res. 46: 2845-2848, 1986
Baruchel S, Viau G. In vitro selective modulation of cellular GSH by a humanized native milk protein isolate in normal cells and rat mammary carcinoma model. Anticancer Res. 16: 1095-1100, 1996
Baruchel S, Viau G, Olivier R, Bounous G, Wainberg M. Nutriceutical modulation of cellular GSH with a humanized native milk serum protein isolate Immunocal: application in AIDS and cancer. In: Oxidative stress in cancer, AIDS and neurodegenerative diseases. Institute Pasteur. Marcel Dekker Inc. 1996
Jimmy Gutman. "GSH. Your Body's Most Powerful Protector Glutathione". 3rd edition. pp. 74-75
Cancer patients should first talk to their treating physician before initiating any glutathione boosting therapies.