carcinogen

A carcinogen is any agent capable of causing cancer — a substance, radiation, or organism that nudges normal cells onto the path of uncontrolled growth. Unlike acute poisoning, its damage is usually slow and silent, sometimes surfacing only years or decades after exposure.

Carcinogens work in two broad ways. Genotoxic carcinogens directly damage DNA or react with it, producing the mutations that can switch on growth-promoting genes or disable tumour-suppressor genes; many are activated to reactive metabolites by the body's own metabolism. Non-genotoxic carcinogens do not mutate DNA directly but promote cancer by other routes — chronic irritation and cell proliferation, hormonal stimulation, or immune suppression.

Cancer development is a multistep process, classically described as initiation (the first mutation), promotion (expansion of the altered cells), and progression. Because it usually takes many hits over a long latency, carcinogenicity is hard to prove in one person and is instead inferred from animal studies, epidemiology, and mechanistic data; agencies such as the International Agency for Research on Cancer grade the strength of that evidence.

In pharmacology there is an uncomfortable paradox: some of the most useful drugs are themselves carcinogenic. Alkylating chemotherapy agents that cure one cancer by damaging DNA can, years later, cause a secondary leukaemia, and long-term immunosuppression raises cancer risk. Here the carcinogenic risk is knowingly accepted because the immediate benefit is far greater.