Chemotherapy

Chemotherapy is a conventional but still controversial treatment for cancer with serious drawbacks. It is typically aggressive and has many side-effects such as: It is a favourite bête noire of alties, who invariably call it "toxic chemotherapy" or "cytotoxic chemotherapy".
 * Easy bruising and bleeding
 * Infection
 * Anemia (low red blood cell counts)
 * Nausea and vomiting
 * Appetite changes
 * Constipation
 * Diarrhea
 * Mouth, tongue, and throat problems such as sores and pain with swallowing
 * Nerve and muscle problems such as numbness, tingling, and pain
 * Skin and nail changes such as dry skin and color change
 * Urine and bladder changes and kidney problems
 * Weight changes
 * Chemo brain, which can affect concentration and focus
 * Mood changes
 * Changes in libido and sexual function
 * Fertility problems

Despite chemotherapy's critics claiming it is only effective in 2%-3% of cases of cancer, there can be no one-number-answer: it depends on the type & stage of cancer, and type of chemotherapy agent, e.g. in childhood-leukemia, chemotherapy provides an 80% survival at 10 years, which would otherwise be 0%.

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.

Technology
Cancer occurs when a cell or group of cells gets broken and starts reproducing like crazy, wreaking havoc on their surroundings, disrupting the flow of bodily fluids and the workings of organs. 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" with "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, also reproduce rapidly and thus are unintentionally affected by chemotherapy. This results in side effects such as sores and ulcers throughout your body, infertility, fatigue, a massively compromised immune system, and baldness. Chemotherapeutical agents are mutagenic, which makes them carcinogenic - this means that they may cause more cancers. 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. These cranks are forgetting one thing: here 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.

Examples
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.