Chemotherapy drugs work primarily by interfering with the cellular machinery of division, targeting processes like DNA replication or cell-cycle progression that every dividing cell has to complete successfully in order to reproduce. Cancer cells divide considerably more rapidly and persistently than most healthy adult cells, so chemotherapy's disruption of the division process hits them especially hard. But that same targeting mechanism doesn't actually distinguish cancerous cells from any other rapidly dividing cell, which is exactly why chemotherapy also damages the body's own fast-dividing healthy cell populations, producing its well-known range of side effects.
Chemotherapy targets a process, not cancer specifically
Rather than recognising some unique molecular marker specific to cancer cells and attacking only cells carrying it, most classic chemotherapy drugs instead target the general biological process of rapid cell division itself, disrupting steps like DNA replication or the cell's ability to complete division successfully. Because cancer cells are defined largely by their loss of normal control over the cell cycle, dividing repeatedly and rapidly in ways healthy cells normally don't, this process-level targeting hits cancer cells especially hard relative to most quiescent, slowly dividing healthy tissue.
Healthy tissues that also divide rapidly bear the collateral damage
The body does contain several genuinely healthy tissues that also divide rapidly under normal conditions, hair follicles, the lining of the digestive tract, and bone marrow producing new blood cells among them, and because chemotherapy's targeting mechanism can't distinguish these healthy rapidly dividing cells from cancerous ones, it damages them as real collateral effect. This is precisely the underlying explanation for chemotherapy's classic, widely recognised side effects, hair loss, digestive symptoms, reduced blood cell counts, each one traceable directly back to a specific healthy tissue whose own normal rapid division made it vulnerable to the same process-level attack targeting the cancer.
What we're still unsure about
The basic mechanism connecting chemotherapy's cell-division targeting to both its cancer-fighting effect and its characteristic side effects is well established, extensively documented pharmacology, confirmed across decades of clinical oncology practice. What remains a genuinely significant, actively worked-on research area is developing more targeted cancer therapies, including immunosuppressant-adjacent and immune-based approaches, capable of attacking cancer cells more specifically while sparing healthy rapidly dividing tissue, since classic chemotherapy's fundamental lack of cancer-specificity is precisely the limitation newer targeted and immunotherapy approaches are trying to overcome — this remains a very active area of ongoing cancer research, without every cancer type yet having an equally effective, more specifically targeted alternative available.
This sits inside Chemotherapy & Immunosuppressants, one of seven topics in Pharmacology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.