Does N-acetylcysteine increase the risk or progression of cancer?

On one hand, excessive reactive oxygen species (ROS) can damage molecules in the body, including DNA, and induce biochemical pathways that may increase the risk of cancer. Additionally, some cancers, like triple-negative breast cancer, use increased ROS signaling for their own survival and progression.^[1] In these contexts, antioxidants such as NAC may play a beneficial role.

On the other hand, the generation of ROS is a defense mechanism the body can use to induce cancer cell death and prevent cancer progression. While antioxidants might protect against ROS-induced damage and reduce cancer progression in some instances,^[2] in other contexts they might promote cancer cell survival by reducing oxidative stress within the cell.^[3] In fact, there’s evidence to suggest that one mechanism by which cancer cells promote their own survival is by increasing the body's antioxidant defense mechanisms within the cell.^[4][5]

There is no evidence from human clinical trials that NAC causes cancer or increases cancer progression. In breast cancer, one uncontrolled pilot study suggested that NAC might actually inhibit cancer cell proliferation.^[2] However, research in mice has suggested that NAC supplementation might promote the progression of certain pre-existing cancers, including skin, liver, and lung cancer, and increase the risk of metastatic disease — in both cases, by reducing oxidative stress in the cancer cells.^[6][7][8][3] In mice without pre-existing cancer, the effects are less clear. NAC supplementation did not induce liver cancer in otherwise healthy mice in one study,^[8] but it did increase the risk of lung cancer in healthy aged mice in another.^[3]

Clearly, the question of how NAC interacts with cancer risk or progression is incredibly nuanced, and more research is needed to understand this relationship.