Chemotherapy drugs are medications that destroy cancer cells by disrupting their ability to grow and divide. They work by targeting the cell cycle, the process every cell goes through to reproduce itself, and interfering with one or more stages of that process. Unlike newer targeted therapies that zero in on specific proteins driving a particular tumor, traditional chemotherapy drugs attack all rapidly dividing cells in the body. That’s what makes them effective against cancer and also what causes their well-known side effects.
How Chemotherapy Drugs Work
Every cell in your body reproduces by moving through a cycle with four stages: a growth phase, a DNA-copying phase (called synthesis), a second growth phase, and finally mitosis, where the cell physically splits into two. Cancer cells move through this cycle faster and more chaotically than normal cells, which is what allows tumors to grow. Chemotherapy drugs disrupt one or more of these stages, either damaging the cell’s DNA so it can’t copy itself, starving it of the raw materials it needs to build new DNA, or physically preventing it from splitting in two.
Different drugs hit different phases of the cycle. That’s why oncologists often combine multiple chemotherapy drugs in a single treatment regimen. By attacking the cell cycle at several points simultaneously, combination therapy kills more cancer cells and makes it harder for the cancer to develop resistance to any one drug.
The Major Classes of Chemotherapy Drugs
Chemotherapy drugs fall into several broad categories based on how they interfere with cell division.
- Alkylating agents were among the first cancer drugs developed and remain the most commonly used class today. They work by directly damaging a cell’s DNA, creating chemical bonds that prevent the DNA strands from separating and copying themselves. They tend to be most effective against slow-growing cancers.
- Antimetabolites mimic the nutrients a cell needs to build new DNA. The cell absorbs them as if they were real building blocks, but because they’re fakes, the cell can’t complete DNA synthesis and essentially starves.
- Plant alkaloids are derived from plants and block a cancer cell’s ability to physically divide into two daughter cells. They interfere with the structural scaffolding inside the cell that pulls it apart during mitosis.
- Antitumor antibiotics (unrelated to the antibiotics that treat infections) bind directly to DNA and cause the tightly coiled strands of genetic material to unwind. This prevents the cell from reading its own genetic instructions to make the proteins it needs to survive and reproduce.
How Chemotherapy Is Given
Most people picture an IV drip when they think of chemotherapy, but these drugs can be delivered in many different ways depending on the type of cancer, the specific drug, and the treatment plan.
Oral chemotherapy comes as pills, capsules, or liquids you swallow at home. Intravenous (IV) chemotherapy can range from a quick injection lasting a few minutes to a continuous infusion running over several days. Some patients carry portable pumps that deliver a slow, steady dose while they go about their daily life.
For longer IV treatment courses, doctors often place a more permanent access device rather than starting a new IV each session. A port (sometimes called a port-a-cath) is a small device implanted under the skin of the chest, connected to a catheter that threads into a large vein near the heart. A nurse accesses it with a special needle through the skin. A PICC line is a long, thin catheter inserted through a vein in the arm. Both options spare patients from repeated needle sticks in the hands and arms.
Some situations call for more specialized delivery. Intrathecal chemotherapy is injected directly into the spinal fluid through a lumbar puncture, allowing drugs to reach cancers in the brain or spinal cord. Intraperitoneal chemotherapy is delivered directly into the abdominal cavity for cancers that have spread across the lining of the abdomen. Intra-arterial chemotherapy sends the drug directly into the artery feeding a tumor, concentrating the dose right where it’s needed.
When Chemotherapy Is Used
Chemotherapy serves different purposes depending on when it’s given in the course of treatment. Neoadjuvant chemotherapy is given before the primary treatment, usually surgery. The goal is to shrink a tumor that is currently too large to remove safely or that’s pressing on vital organs. By reducing the tumor first, surgery becomes less extensive and more likely to succeed.
Adjuvant chemotherapy is given after surgery or radiation. Its purpose is to hunt down and kill any microscopic cancer cells that may have been left behind, reducing the chance the cancer comes back. In some advanced cancers, chemotherapy is used as the main treatment to slow disease progression and manage symptoms even when a cure isn’t the realistic goal.
Many treatment plans combine chemotherapy with surgery, radiation, or newer drug therapies. The specific combination depends on the cancer type, its stage, and how the patient responds over time.
How Doses Are Calculated
Chemotherapy dosing isn’t one-size-fits-all. Most doses are calculated based on your body surface area, a measurement derived from your height and weight. This approach accounts for the fact that larger bodies process and clear drugs differently than smaller ones, helping to balance effectiveness against toxicity. Your oncology team also factors in kidney and liver function, age, other medications, and overall health when determining the right dose. Doses are frequently adjusted throughout treatment based on how your body is tolerating the drugs.
Why Side Effects Happen
The fundamental limitation of traditional chemotherapy is that it can’t distinguish between cancer cells and normal healthy cells that also divide rapidly. Your body has several types of fast-dividing cells that get caught in the crossfire.
Bone marrow cells, which produce your blood supply, are particularly vulnerable. When chemotherapy damages these cells, your white blood cell count drops, weakening your immune system and increasing your risk of infection. A condition called neutropenia, where infection-fighting white blood cells fall dangerously low, is one of the most serious short-term risks of treatment. Platelet counts can also drop, making you bruise more easily and bleed longer from minor injuries.
Hair follicles are among the fastest-dividing cells in the body, which is why hair loss is so common during treatment. Cells lining the mouth and digestive tract also divide rapidly, leading to mouth sores, nausea, and digestive problems. Skin cells and reproductive cells are similarly affected.
Some chemotherapy drugs can also cause peripheral neuropathy, a condition where nerves in the hands and feet become damaged. This shows up as numbness, tingling, or a “pins and needles” sensation, and it can make fine motor tasks like buttoning a shirt difficult. Neuropathy sometimes improves after treatment ends, but for some patients it persists long-term.
Most side effects are temporary because healthy cells repair themselves more efficiently than cancer cells. That’s the biological advantage chemotherapy exploits: cancer cells are worse at fixing the damage, so they die while normal cells eventually recover between treatment cycles.
How Chemotherapy Differs From Targeted Therapy
Newer cancer treatments called targeted therapies work by a fundamentally different principle. Instead of attacking all rapidly dividing cells, targeted therapies interfere with specific proteins that help a particular tumor grow and spread. Because they’re more selective, they generally cause different (and often fewer) side effects than traditional chemotherapy, though they come with their own set of risks.
Targeted therapies only work when a tumor has the specific molecular feature the drug is designed to block, so they aren’t an option for every patient or every cancer type. Traditional chemotherapy remains a cornerstone of cancer treatment precisely because of its broad mechanism. It works across a wide range of cancers and can be combined with targeted therapies, immunotherapy, and other approaches to build a more effective overall treatment plan.