Reactive Oxygen Species (ROS) refers to the highly reactive substances which contain oxygen radicals.
Hypochlorous acid, peroxides, superoxide, singlet oxygen, alpha-oxygen, and hydroxyl radicals are the major examples
of ROS. Generally, the reduction of oxygen (O2) in molecular form produces superoxide (•O2
−) anion. ROS are
produced during a variety of biochemical reactions within the cell organelles, such as endoplasmic reticulum, mitochondria,
and peroxisome. Naturally, ROS are also formed as a byproduct of the normal metabolism of oxygen. The
production of ROS can be induced by various factors such as heavy metals, tobacco, smoke, drugs, xenobiotics, pollutants,
and radiation. From various experimental studies, it is reported that ROS acts as either a tumor-suppressing or a
tumor-promoting agent. The elevated level of ROS can arrest the growth of tumors through the persistent increase in
cell cycle inhibition. The increased level of ROS can induce apoptosis by both intrinsic and extrinsic pathways. ROS is
considered to be a tumor-suppressing agent as the production of ROS is due to the use of most of the chemotherapeutic
agents in order to activate cell death. The cytotoxic effect of ROS provides impetus towards apoptosis, but in higher
levels, ROS can cause initiation of malignancy that leads to uncontrolled cell death in cancer cells. In contrast, some
species of ROS can influence various activities at the cellular level, including cell proliferation. This review highlights
the genesis of ROS within cells by various routes and their role in cancer therapies.