Cimetidine to Treat Cancer - Quick Reference Sheet
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Cimetidine to Treat Cancer

Created on 09/08/2026 – Quick Reference based on Evidence Review created using AI4L / Opus 5 – Audit

A cheap, decades-old stomach-acid medicine that also acts on immune cells and the lining of blood vessels. The strongest signal is longer survival around surgery for bowel cancer, but the one later trial set up to confirm it found nothing. Breast tissue growth and sexual side effects fall on men; slowed handling of other medicines is the everyday harm. (Full Review)

Protocol

Perioperative short course
400 mg twice daily
About 5 days before tumour resection, continuing about 2 days afterwards
Adjuvant twelve-month course
800 mg daily
Two weeks after curative colorectal resection, with fluorouracil-based chemotherapy, for 12 months
Best time of day
Split morning and evening
Keeps blood levels more even; a single daily dose is conventionally taken in the evening
Time to effect
Survival endpoint
Years
No perceptible effect; the endpoint takes years to read out
Immune-cell changes
5–7 days
The changes seen in trials were measured on resected tissue
Acid suppression
Within an hour
The approved acid effect, not an anti-tumour readout

Benefits

Contraindications
  • Severe kidney impairment (estimated filtration below 30 mL/min/1.73 m²) unless the dose is at least halved and mental status monitored
  • Decompensated cirrhosis (Child-Pugh Class C)
  • Men with pre-existing gynaecomastia, prostate cancer on androgen-deprivation therapy, or low testosterone
  • Documented hypersensitivity to cimetidine or another H2 blocker
  • Oral tyrosine kinase inhibitors dependent on gastric acid, unless dosing is separated and levels monitored
  • Pregnancy or breastfeeding
  • Adults over 75 with cognitive impairment or delirium risk
    •
Key Interactions
  • Vitamin K antagonists (warfarin, acenocoumarol)
  • Anticonvulsants (phenytoin, carbamazepine)
  • Methylxanthines (theophylline, aminophylline)
  • Antiarrhythmics and beta-blockers (lidocaine, procainamide, propranolol, metoprolol)
  • Clopidogrel
  • Tamoxifen
  • Metformin
  • Liver-cleared benzodiazepines (diazepam, chlordiazepoxide)
  • Over-the-counter antacids and sucralfate
  • Over-the-counter analgesics (paracetamol, ibuprofen, naproxen)
  • Alcohol
  • Caffeine-containing supplements
  • Enzyme-inhibiting supplements (grapefruit juice, piperine, berberine, quercetin, goldenseal)
  • Enzyme-inducing supplements (St John's wort)
  • Other acid-reducing interventions (omeprazole, famotidine, nizatidine)
    •

Risk & Side Effects

  • High: Gynaecomastia and breast tenderness in men; reversible elevation of serum creatinine; interaction-driven toxicity from enzyme and transporter inhibition
  • Medium: Confusion and sedation in older or renally impaired adults
  • Low: Reduced absorption of pH-dependent oral cancer drugs; reversible erectile dysfunction and reduced libido; blunted immune checkpoint inhibitor response — conflicted; blood-count suppression; association between long-term acid suppression and gastric cancer — conflicted; liver enzyme elevation and reversible hepatitis
  • Speculative: Impaired testicular function
    •

Monitoring

Marker Target Why
Serum creatinine 0.7–1.1 mg/dL (men), 0.6–0.9 mg/dL (women) Anchors the expected drug-induced rise
Estimated glomerular filtration rate Above 60 mL/min/1.73 m²; dose halved below 30 Governs dosing; clearance is renal
Complete blood count with differential Neutrophils 1.8–7.0 ×10⁹/L; platelets 175–400 ×10⁹/L Catches rare marrow effects
Alanine aminotransferase Below 25 U/L (men), below 20 U/L (women) Detects uncommon reversible hepatitis
Total testosterone (men) 550–900 ng/dL Tracks the antiandrogenic effect
Prolactin (men) Below 10 ng/mL Rises and drives breast tissue growth
Vitamin B12 500–900 pg/mL Acid suppression beyond six months impairs absorption
International normalised ratio (INR) 2.0–3.0 on warfarin; otherwise 0.9–1.1 Cimetidine slows warfarin clearance
Carcinoembryonic antigen Below 3 ng/mL in non-smokers; otherwise the post-operative baseline Colorectal recurrence surveillance
Tumour sialyl Lewis X and A staining No numeric target; reported high or low on the resected specimen The one biomarker with randomised evidence predicting benefit

Cadence: Baseline before the first dose; creatinine and blood counts at 2 and 6 weeks, then every 3 months across a twelve-month course; interacting drug levels or clotting times at 3–5 days and 2 weeks after any dose change; endocrine markers in men at 6 months.

Qualitative Assessment

  • Mental clarity and orientation, especially in the first fortnight and in anyone over 65
  • Daytime drowsiness, dizziness and sleep quality
  • Breast tenderness, swelling or nipple sensitivity in men
  • Libido and erectile function in men
  • Digestive tolerance: nausea, constipation, diarrhoea or flatulence
  • Energy levels and exercise tolerance across a long course
  • Any new bruising, bleeding or unusual infection, which would prompt an unscheduled blood count
Not medical advice. Educational distillation of a single evidence review. Confirm any decision with a qualified clinician, especially in the presence of prescription drugs or chronic conditions.