cytotoxic drug

A cytotoxic drug is a medicine designed to kill cells that are dividing quickly. Imagine a tool that targets anything growing fast in the body; cancer cells multiply faster than most healthy cells, so they take the heaviest hit. The trade-off is that some normal tissues also divide rapidly, so they suffer collateral damage.

Mechanistically, cytotoxic drugs interfere with the machinery of cell division and DNA, the cell's instruction manual. By damaging DNA, blocking the building blocks needed to copy it, or jamming the spindle that pulls chromosomes apart, they push dividing cells toward death. The classic chemotherapy families (alkylating agents, antimetabolites, antimitotics and topoisomerase inhibitors) are all cytotoxic drugs working through different points of this machinery.

Because they cannot perfectly tell cancer from healthy tissue, cytotoxic drugs predictably harm the fastest-renewing normal cells: bone marrow, gut lining, hair follicles. This is why classic chemotherapy causes low blood counts, mouth sores, nausea and hair loss. Dosing is a constant balance between killing enough tumour and sparing enough normal tissue to recover.