Cytotoxicity refers to the quality or characteristic of being toxic, or harmful, to cells. This can occur when a substance is introduced into the body and has an adverse effect on normal cell functioning, leading to damage or death of the affected cells. Cells are the basic building blocks of living organisms, so anything that harms them can have serious consequences for overall health and well-being. In medical terms, cytotoxicity is often used in relation to drugs or other treatments that have a negative impact on cellular function, which may be beneficial in certain situations (such as killing cancer cells) but can also cause unwanted side effects.