Chemotherapy is used primarily to treat cancer, but it also plays a role in managing certain autoimmune diseases and preparing the body for bone marrow transplants. In cancer treatment, chemotherapy drugs work by disrupting the ability of fast-growing cells to divide and multiply. Depending on the type and stage of cancer, chemotherapy can be the main treatment or one piece of a larger plan that includes surgery, radiation, or newer therapies like immunotherapy.
How Chemotherapy Works in the Body
Cancer cells grow and divide faster than most normal cells. Chemotherapy drugs exploit this by interfering with the machinery cells use to copy their DNA and split into new cells. Some drugs block the raw materials a cell needs to build new DNA. Others damage the DNA strands directly, making it impossible for the cell to replicate. A third category stops cells from physically pulling apart during the final stage of division.
Because these drugs travel through your bloodstream, they reach cancer cells throughout the body, not just at one site. That’s what makes chemotherapy especially valuable for cancers that have spread or for blood cancers that don’t form solid tumors. The tradeoff is that chemotherapy also affects healthy cells that naturally divide quickly, including cells in your hair follicles, bone marrow, skin, and the lining of your digestive tract. That’s why hair loss, nausea, and lowered immunity are among the most common side effects.
The Four Goals of Chemotherapy
Not every round of chemotherapy has the same purpose. The goal depends on the type of cancer, how advanced it is, and what other treatments are involved.
Curative treatment aims to eliminate the cancer entirely. This is most realistic with certain blood cancers and germ cell tumors (like testicular cancer), where chemotherapy alone can produce a cure in a significant proportion of patients. For most solid tumors, chemotherapy rarely cures cancer on its own but becomes curative when combined with surgery or radiation.
Adjuvant therapy is chemotherapy given after surgery or radiation to kill any remaining cancer cells that might be too small to detect. Breast cancer treatment, for example, frequently includes adjuvant chemotherapy to reduce the chance of the cancer returning.
Neoadjuvant therapy flips that order. Chemotherapy is given before surgery to shrink a tumor, making it easier to remove or, in some cases, making surgery possible when it otherwise wouldn’t be. This approach is also used as an organ preservation strategy, helping patients with cancers of the throat, bladder, or rectum avoid the removal of those organs.
Palliative treatment is used when a cure isn’t the goal. Here, chemotherapy aims to slow the cancer’s growth, shrink tumors that are causing pain or other symptoms, and improve quality of life. In some situations, palliative chemotherapy also extends survival. In metastatic prostate cancer, for instance, patients who received chemotherapy had 30-month survival rates of about 62%, compared to 54% for those who did not.
Cancers Commonly Treated With Chemotherapy
Chemotherapy is used across a wide range of cancer types, though how central a role it plays varies considerably. Blood cancers like leukemia, lymphoma, and multiple myeloma rely heavily on chemotherapy because there’s no tumor to cut out surgically. Testicular cancer is one of the most chemotherapy-responsive solid tumors, with high cure rates even in advanced stages.
For breast, lung, colorectal, and ovarian cancers, chemotherapy is typically part of a combination strategy. It might be given before surgery to shrink a tumor, after surgery to prevent recurrence, or as the primary treatment when cancer has spread to distant organs. The specific drugs and the number of cycles vary depending on the cancer’s biology and how it responds.
Chemotherapy Beyond Cancer
Several chemotherapy drugs are used at lower doses to treat autoimmune diseases, where the immune system mistakenly attacks the body’s own tissues. Conditions like lupus, rheumatoid arthritis, multiple sclerosis, and certain blood disorders can be managed with drugs originally developed for cancer. The principle is similar: these medications suppress the overactive immune cells that are driving the disease. The doses used for autoimmune conditions are generally much lower than those used in cancer treatment, which means fewer and milder side effects.
Chemotherapy also serves a critical role in bone marrow and stem cell transplants. Before a transplant, high-dose chemotherapy is given to wipe out the patient’s existing bone marrow. This accomplishes two things: it reduces the cancer burden (if the transplant is for a blood cancer) and it suppresses the immune system enough to prevent the body from rejecting the new donor cells. These conditioning regimens typically use combinations of drugs chosen specifically because their most potent effects target marrow cells.
How Chemotherapy Is Given
The most common delivery method is intravenous infusion, where the drugs flow directly into a vein. For patients who need many rounds of treatment, a port is often surgically placed in the chest. This small device sits just under the skin and connects to a large vein, giving nurses easy access for each session without repeatedly searching for a vein in the arm.
Some chemotherapy comes in pill form and can be taken at home. This is increasingly common for certain cancers and maintenance regimens. Other, less common routes include injections directly into the fluid around the spinal cord (for cancers affecting the brain or nervous system) and topical creams for specific skin cancers. Where you receive treatment depends on the method: IV infusions usually happen in an outpatient clinic or hospital, while oral chemotherapy fits into your normal routine at home.
Chemotherapy Combined With Newer Treatments
Chemotherapy is increasingly paired with immunotherapy, which helps the immune system recognize and attack cancer cells. This combination, sometimes called chemo-immunotherapy, has become a standard approach for several difficult-to-treat cancers. More than 200 clinical trials have tested combinations of chemotherapy with immune checkpoint inhibitors, and multiple regimens have received regulatory approval based on meaningful improvements in survival.
The results can be striking. In one example involving advanced stomach cancer that overproduces a protein called HER2, adding immunotherapy and a targeted drug to chemotherapy boosted the response rate from about 52% to 74%. The logic behind pairing these treatments is straightforward: chemotherapy kills cancer cells directly and can also make tumors more visible to the immune system, while immunotherapy sustains the immune response over time. The side effects of each treatment don’t typically overlap in dangerous ways, which makes the combination manageable for most patients.
What a Treatment Cycle Looks Like
Chemotherapy is given in cycles, with treatment days followed by rest periods that allow your body to recover. A typical cycle lasts two to three weeks, though some regimens use weekly or biweekly schedules. During the rest period, healthy cells in your bone marrow and digestive tract regenerate, which is why blood counts are checked before each new cycle to confirm your body is ready.
A full course of chemotherapy might involve four to eight cycles, stretching over several months. The exact number depends on the cancer type, the treatment goal, and how well the cancer responds. Your oncologist monitors progress through imaging scans and blood tests, adjusting the plan if the cancer isn’t responding or if side effects become too severe. Some people continue a lower-dose maintenance regimen for months or even years after the initial treatment course to keep the cancer from returning.