Chemotherapy: How It Works, Side Effects, and What to Expect

Chemotherapy is the use of systemic drugs that kill rapidly dividing cells — including cancer cells — to treat malignancy. It remains one of the most widely used cancer treatments, both as a standalone modality and in combination with surgery, radiation, targeted therapy, and immunotherapy. Chemotherapy is fundamentally different from targeted therapy, which attacks specific molecular alterations driving a particular tumor’s growth. Chemotherapy, by contrast, acts broadly on all rapidly dividing cells — cancer cells divide faster than most normal cells, making them more vulnerable, but normal rapidly dividing tissues (bone marrow, gut epithelium, hair follicles) are also affected.

This guide covers what chemotherapy is, how each drug class disrupts cancer cell biology, which regimens are used for which cancers, how side effects are managed with modern supportive care, and what to expect during treatment. For a complete overview of all cancer treatment modalities, see the cancer treatment guide. For guidance on how to choose between chemotherapy and other options, see the cancer treatment options guide.

How Chemotherapy Works — Drug Classes

Chemotherapy drugs disrupt cancer cell replication through several distinct mechanisms. Understanding these mechanisms helps explain both their effectiveness and their specific side effect profiles.

Key Mechanism Details

5-FU and leucovorin: 5-FU inhibits thymidylate synthase (TS), depleting thymidine for DNA synthesis. Leucovorin (folinic acid) stabilizes the 5-FU/TS inhibitory complex, making 5-FU significantly more effective — which is why FOLFOX, FOLFIRI, and FLOT always include leucovorin. Continuous IV infusion of 5-FU over 46 hours provides sustained TS inhibition superior to IV bolus. Capecitabine (Xeloda) is an oral prodrug converted to 5-FU preferentially within tumor tissue, allowing outpatient oral administration in place of IV infusion.

Temozolomide and MGMT: TMZ, the oral alkylator used in glioblastoma (Stupp protocol: concurrent RT + TMZ → adjuvant TMZ), methylates the O6 position of guanine. Tumors with MGMT promoter methylation cannot express the MGMT repair enzyme that reverses this damage — making them significantly more sensitive to TMZ. MGMT status is a key biomarker tested before glioblastoma treatment.

Irinotecan and UGT1A1: Irinotecan is metabolized to its active form SN-38 by the UGT1A1 enzyme. Patients with the UGT1A1*28 polymorphism (the Gilbert syndrome genotype, ~10% of the population) metabolize SN-38 more slowly — accumulation leads to severe diarrhea and neutropenia at standard doses. UGT1A1 genotyping is recommended before irinotecan at doses ≥250 mg/m². For more on how targeted therapies differ mechanistically from chemotherapy, see the targeted therapy guide.

Common Chemotherapy Regimens by Cancer Type

The Chemotherapy Cycle — What Happens

A chemotherapy cycle is a scheduled treatment period followed by a recovery period, repeated until the planned number of cycles is complete. Recovery time allows the bone marrow to replenish blood cells before the next cycle — cancer cells, which repair less efficiently, have less opportunity to recover.

Common cycle lengths:

• Every 2 weeks (q2w): FOLFOX, FOLFIRI, FOLFIRINOX (treatment day + 46h 5-FU pump = 3 days, then 11 days off)
• Every 3 weeks (q3w): AC (breast cancer), BEP (testicular), carboplatin/paclitaxel
• Every 4 weeks: Many oral regimens (capecitabine: 14 days on, 7 days off)

What happens at each cycle visit:

1. Pre-cycle bloodwork: CBC (ANC ≥1,500/μL required; platelets ≥100,000/μL); CMP (creatinine for carboplatin Calvert formula: Dose mg = Target AUC × [CrCl + 25]; bilirubin for irinotecan risk)
2. Nursing assessment: Weight, symptom review, performance status
3. Pre-medications: Antiemetics before chemotherapy starts; dexamethasone + antihistamine for taxane hypersensitivity prevention; G-CSF counseling
4. Chemotherapy infused: Via peripheral vein, PICC line, or implanted port (port preferred for multi-cycle regimens — preserves peripheral veins)
5. 5-FU pump (if applicable): Patient goes home with a portable pump; 46-hour continuous 5-FU infusion; pump disconnected at day 3

The nadir: Blood counts reach their lowest point — the nadir — approximately 10–14 days after chemotherapy for most agents (carboplatin nadir: ~21 days). This is the window of highest infection risk. Patients should check their temperature daily during the nadir and know clearly: fever ≥38.3°C (100.4°F) = go to the ER immediately. For a comprehensive list of questions to ask your team before starting chemotherapy, see the cancer diagnosis questions guide.

Managing Chemotherapy Side Effects

Myelosuppression and Febrile Neutropenia

Chemotherapy suppresses the bone marrow’s production of all blood cells:

• Neutropenia: ANC nadir 10–14 days post-treatment; febrile neutropenia (FN) occurs when ANC <500/μL + fever ≥38.3°C
• Thrombocytopenia: Most pronounced with carboplatin and oxaliplatin; platelet transfusion threshold typically <10,000/μL
• Anemia: Iron supplementation if iron-deficient; transfusion for symptomatic severe anemia

G-CSF (Granulocyte Colony-Stimulating Factor) prevents FN by stimulating neutrophil production from bone marrow:

• Filgrastim (Neupogen): Daily subcutaneous injections, started 24–72 hours after chemotherapy completion
• Pegfilgrastim (Neulasta): Single injection per cycle — equivalent efficacy; preferred for dose-dense every-2-week regimens

G-CSF is required when FN risk ≥20% (dose-dense AC-T, FOLFOXIRI, ABVD in elderly) and strongly considered for 10–20% risk with patient risk factors (age >65, PS ≥2, prior FN, no previous G-CSF).

Nausea and Vomiting (CINV)

CINV is the most feared chemotherapy side effect — and now the most reliably prevented with modern antiemetics. The key rule: take antiemetics before chemotherapy starts, not after nausea begins.

Olanzapine 10 mg added to standard antiemetic regimens for HEC significantly improves complete CINV control — endorsed by MASCC/ESMO guidelines. Palonosetron (second-generation 5-HT3 receptor antagonist with 40-hour half-life) is preferred over first-generation agents (ondansetron, granisetron) for both acute and delayed CINV prevention.

Peripheral Neuropathy

• Oxaliplatin: Causes acute cold-related dysesthesia — tingling in fingertips and throat triggered by cold exposure (ice, cold drinks, cold surfaces) lasting ~1 week after each infusion. Patients must avoid cold during this window. Cumulative sensorimotor neuropathy develops with successive cycles and may persist after treatment ends.
• Paclitaxel: Cumulative distal sensory neuropathy — numbness and tingling beginning in fingertips and toes, progressing proximally with each cycle. May be dose-limiting.
• Vincristine: Peripheral and autonomic neuropathy (constipation, foot drop); does not cause myelosuppression.

Management: Dose reduction is the primary intervention. Duloxetine (SNRI antidepressant) has ASCO-endorsed moderate evidence for established oxaliplatin or paclitaxel neuropathy — the only pharmacologic intervention with randomized trial support (Smith EM et al., JAMA 2013).

Additional Side Effects

Alopecia: Expected with anthracyclines, taxanes, irinotecan, and cyclophosphamide — hair loss typically begins 2–3 weeks after the first cycle. Scalp cooling (DigniCap, Paxman — FDA-cleared) reduces alopecia in 50–66% of breast cancer patients receiving taxane-based therapy. Not appropriate for hematologic malignancies.

Mucositis: Painful mouth sores and GI inflammation with 5-FU-based regimens, high-dose methotrexate, and cytarabine. Prevention: meticulous oral hygiene, avoidance of alcohol-containing mouthwash, and palifermin (approved for mucositis in hematologic malignancy high-dose regimens).

Cardiotoxicity (anthracyclines): Baseline ECHO or MUGA scan required; monitor during treatment at cumulative dose thresholds (doxorubicin ~250 mg/m², ~400 mg/m²). Avoid concurrent anthracycline + trastuzumab — dramatically increased cardiotoxicity risk.

Cisplatin nephrotoxicity: Dose-dependent renal tubular damage; requires aggressive pre-hydration (1–2L IV saline + mannitol before and after). Carboplatin (dosed by Calvert formula based on renal function) is far less nephrotoxic and preferred when eGFR is borderline.

Hemorrhagic cystitis (ifosfamide, high-dose cyclophosphamide): Caused by acrolein metabolite; MESNA uroprotection is mandatory with each ifosfamide dose.

Bloodwork and Monitoring During Chemotherapy

Standard labs before each cycle confirm it is safe to proceed:

• CBC with differential: ANC ≥1,500/μL; platelets ≥100,000/μL; hemoglobin for transfusion threshold
• Creatinine/eGFR: Critical for carboplatin Calvert formula dosing; cisplatin typically avoided if eGFR <50 mL/min
• LFTs and bilirubin: Elevated bilirubin signals reduced irinotecan metabolism → SN-38 accumulation → dose reduction required
• Magnesium: Cisplatin causes renal magnesium wasting; monitor and replace to prevent hypomagnesemia-related arrhythmias
• UGT1A1 genotype: Before high-dose irinotecan; *28/*28 homozygotes require dose reduction
• Cardiac monitoring: Baseline ECHO before anthracyclines; repeat at cumulative dose thresholds

Understanding cancer stage is prerequisite to most chemotherapy decisions — stage determines intent (curative vs. palliative), regimen intensity, and planned number of cycles. According to the NCCN Patient Guidelines, chemotherapy regimen selection should be reviewed by a multidisciplinary oncology team including medical oncologist, pharmacist, and oncology nurse coordinator.

Frequently Asked Questions

• National Cancer Institute — Chemotherapy to Treat Cancer
• American Cancer Society — Chemotherapy for Cancer
• Chemocare.com — Patient Drug Information
• Conroy T et al. — PRODIGE 4 (FOLFIRINOX vs. gemcitabine in pancreatic cancer); NEJM 2011
• Gandhi L et al. — KEYNOTE-189 (pembrolizumab + chemo for NSCLC); NEJM 2018
• Citron ML et al. — CALGB 9741 (dose-dense AC-T); JCO 2003
• Coiffier B et al. — R-CHOP for DLBCL; NEJM 2002
• Connors JM et al. — ECHELON-1 (BV-AVD vs. ABVD in advanced HL); NEJM 2018
• Smith EM et al. — Duloxetine for chemotherapy-induced neuropathy; JAMA 2013
• Moore K et al. — SOLO1 (olaparib maintenance in ovarian cancer); NEJM 2018

This article is for educational purposes only and does not constitute medical advice. Discuss all chemotherapy decisions with your oncology care team.