Cimetidine is an inexpensive, familiar stomach-acid medicine studied as an add-on in cancer, mainly around colon and rectal surgery. Given during that window, it may help some patients live longer, with the strongest signal in tumors carrying certain sugar markers. The evidence is genuine but unproven; its main drawback is slowing the breakdown of many other medicines. (Full Review)

Protocol

Dose

800 mg/day

Oral; commonly 400 mg twice daily

Timing

Perioperative

Started a few days before surgery, continued through the postoperative period

Duration

Days to 1 year

Short course ~5 days spanning surgery, or extended adjuvant use

Time to effect

Survival benefit

Years

Long-horizon outcome, not something an individual can feel

Immune changes

Days to weeks

Measured shifts in tumor-infiltrating and circulating immune cells

Acid effects

Hours

Acid-lowering action, separate from the anticancer rationale

Benefits

High: Improved survival after colorectal cancer surgery in sialyl Lewis–positive tumors

Medium: Overall perioperative survival signal in resected colorectal cancer; preserved anti-tumor immunity and increased tumor-infiltrating lymphocytes

Low: Reduced metastatic spread via adhesion blockade; perioperative survival signal in gastric cancer; adjunct role in renal cell carcinoma and immunotherapy regimens

Speculative: Chemoprevention of certain cancers; activity in melanoma and other solid tumors; synergy with modern chemotherapy and immunotherapy

Contraindications

Dofetilide

Pregnancy and breastfeeding

Advanced liver disease (Child-Pugh Class C)

Multiple narrow-therapeutic-index drugs

Key Interactions

Anticoagulants and antiplatelets (warfarin, clopidogrel)

Anticonvulsants (phenytoin, carbamazepine)

Theophylline and lidocaine

Antiarrhythmics (procainamide, quinidine, propranolol)

Metformin and other renally cleared cations

Sedatives and antidepressants (diazepam, amitriptyline, imipramine, codeine, hydrocodone)

Antacids and sucralfate

Iron and vitamin B12

Some chemotherapies (fluorouracil, etoposide)

Risk & Side Effects

High: Cytochrome P450 drug interactions

Medium: Anti-androgen effects; central-nervous-system effects in the elderly and in kidney impairment

Low: Benign rise in serum creatinine; common minor effects; rare hematologic and hepatic effects

Speculative: Possible increased prostate cancer risk in older men; rebound acid hypersecretion and symptom masking; unknown long-term effects at high anticancer doses

Monitoring

Marker	Target	Why
Serum creatinine / eGFR	eGFR >90 mL/min/1.73m²; creatinine within lab norm	Guides dosing and interaction risk
INR (if on warfarin)	2.0–3.0 (target-dependent)	Detects bleeding risk from CYP2C9 inhibition
Complete blood count (CBC)	Within lab reference range	Screens for rare reversible cytopenias
Liver enzymes (ALT/AST)	ALT <25 U/L (men), <20 U/L (women)	Screens for uncommon transient enzyme rise
Tumor sialyl Lewis-X / -A expression	High expression favors benefit	Candidate predictor of who responds
Testosterone / prolactin	Testosterone within age-appropriate range	Flags anti-androgen effect early

Cadence: Check interacting-drug levels (such as INR) within 3–5 days of starting and after any dose change; review kidney function and blood counts every 3–6 months during sustained use; reassess cognition at each visit in older adults

Qualitative Assessment

Cognitive clarity and absence of new confusion, especially in older adults
Energy levels and fatigue
Breast tenderness or swelling and sexual function in men on prolonged high-dose use
Digestive comfort and any signs of nutrient shortfall

Cimetidine to Treat Cancer

Protocol

Benefits

Risk & Side Effects

Monitoring

Qualitative Assessment