Cancer cells divide fast. Chemotherapy drugs target that speed, interfering with DNA replication or cell division. The problem is that some healthy cells also divide fast: hair follicles, the lining of the gut, and blood cells in the bone marrow. That overlap explains most of the side effects, from hair loss to nausea to low blood counts.

The drugs come in several classes. Alkylating agents like cyclophosphamide damage DNA directly. Antimetabolites like methotrexate mimic building blocks and disrupt synthesis. Topoisomerase inhibitors break the enzymes that untangle DNA. Taxanes stabilize microtubules and block cell division. Most treatment regimens combine several drugs to hit cancer cells at different points and reduce the chance of resistance. They are given in cycles, with rest periods for healthy tissue to recover.

What patients experience:

• Fatigue. The most common complaint, often lasting weeks.
• Nausea. Modern antiemetics have made this far more manageable.
• Hair loss. Temporary, regrows after treatment ends.
• Infection risk. Low white counts mean fevers need immediate attention.

Targeted therapies and immunotherapy have changed cancer treatment, but chemotherapy remains essential for many cancers, especially when used before or after surgery.