Chemotherapy is a conventional treatment for cancer. It is typically aggressive and has many side effects; it is a favourite bête noire of alties, who invariably call it "toxic chemotherapy" or "cytotoxic chemotherapy" (both accurate and therefore pointless, the dolts do not realise that these are merely technical terms for how it works).
With advances in surgery, radiotherapy and gene-targeted cancer treatments, the additional differential effect of chemotherapy is being eroded. It is quite likely to decline in use over the coming decades. Although alties claim that cancer doctors love the "cut, poison, burn" approach of surgery, chemo and radiotherapy, nobody has ever met an oncologist who actually likes chemo. Most of them will be at the front of the ticker tape parade when the alties and quacks finally get round to publishing evidence that they actually have a non-aggressive cure for cancer.
Cancer occurs when a cell or group of cells gets broken and starts reproducing like crazy, wreaking havoc on their surrounding, disrupting the flow of bodily fluids and the workings of organ. This is bad. What makes treating cancer hard is targeting a cancer cell because they are by definition just like the rest of the cells around. The only major difference is that they are reproducing chaotically and wrecking shit up.
Chemotherapy acts in a way that is
deceptively simple : it affects cells that reproduce quickly. As such, it replaces an ablation's "kill everything that moves" by "kill everything that runs". This, obviously, is not a very subtle or consequence-free way to do things, which is why nobody likes it.
Cancerous cells are obviously not the only cells that reproduce quickly. Cells such as the lining of your digestive system (from your mouth to your rectum), reproductive cells, bone marrow, hair follicles, and a few others. This results in sores and ulcers throughout your body, infertility, fatigue, massively compromised immune system, and baldness. Since the cure is practically a disease itself, this leads a lot of people to desperately seek alternatives no matter how dubious, creating a goldmine for woo. But in the real world, if Chemo wasn't the best option for the situations in which it's used, modern medicine wouldn't use it.
See how many food woo lies you can find in this appalling example of a supposed "alternative to chemotherapy": "Natural Alternatives to Chemotherapy" Please, if you are looking for real information, do not follow the advice here without consulting a proper oncologist.