How does chemotherapy actually work?
By damaging cells that are dividing rapidly, which is what cancer cells do — and also what several healthy tissues do, which is where essentially all the side effects come from.
The mechanism. Traditional chemotherapy drugs interfere with cell division: damaging DNA so it cannot be copied, blocking the building blocks needed to make new DNA, or disrupting the machinery that pulls chromosomes apart. A cell attempting to divide with that damage typically dies. Cancer cells divide more often than most healthy cells, so they are hit harder — but the targeting is a matter of degree, not precision.
Which healthy tissues divide rapidly, and therefore what side effects occur:
Bone marrow — producing blood cells. Hence low white cells and infection risk, anaemia and fatigue, and low platelets and bruising. Neutropenic sepsis is the serious one, and a fever during treatment is an emergency.
The gut lining — nausea, mouth ulcers, diarrhoea.
Hair follicles — hair loss, which is temporary and does not occur with every drug.
Skin and nails.
Why it is given in cycles. Treatment is spaced to let healthy tissue recover while cancer cells, which recover less effectively, accumulate damage. This is the reason for the rest weeks, and why skipping them is counterproductive.
Why drugs are combined. Different drugs attack different points of the cell cycle, which raises effectiveness and makes resistance less likely — the same logic as combination antibiotics.
The other treatment types, frequently confused with chemotherapy:
Targeted therapy blocks a specific molecular abnormality present in the cancer, with a different and usually narrower side-effect profile.
Immunotherapy removes the brakes cancers use to evade the immune system, so the body attacks the tumour. Side effects are autoimmune in character rather than related to cell division.
Hormone therapy starves cancers that depend on hormones.
Radiotherapy damages DNA in a defined area only.
Why it is used when a cure is not expected — to shrink tumours, relieve symptoms and extend life, which is a legitimate and common goal.
General information, not medical advice.