H2 blockers, used to treat gastrointestinal ulcers and/or reduce stomach acid, may improve survival among people with cancer, especially after colorectal cancer resection.
Safety and precautions
Cimetidine has low toxicity overall; however, several cautions, side effects, and interactions with other drugs are noted. Proton-pump inhibitors (PPIs) are the most commonly prescribed drugs to treat stomach-acid issues,1Gardner A. Should you be afraid of stomach-acid medicine? Penn Medicine News. July 02, 2019. Viewed May 1, 2023. but H2 blockers are also commonly used to reduce stomach acid. Because they are often used without a prescription, their use is not always monitored by a health professional. We strongly recommend that you tell your oncologist if you are using H2 blockers.
Cancer risk
Increased risk of breast cancer, squamous cell carcinoma, or adenocarcinoma and are noted among people using H2 blockers. No rise in esophageal or stomach (gastric) cancer has been seen.
Interactions with cancer treatment
Use of either proton pump inhibitors or H2 blockers can also lead to an inaccurate fasting plasma gastrin assay for diagnosing a gastrinoma.10Dhillo WS, Jayasena CN et al. Plasma gastrin measurement cannot be used to diagnose a gastrinoma in patients on either proton pump inhibitors or histamine type-2 receptor antagonists. Annals of Clinical Biochemistry. 2006 Mar;43(Pt 2):153-5.
Common side effects/adverse events
Common side effects include headache, dizziness, diarrhea, and rash. Other less common but more serious side effects include these:11Pantziarka P, Bouche G, Meheus L, Sukhatme V, Sukhatme VP. Repurposing drugs in oncology (ReDO)—cimetidine as an anti-cancer agent. Ecancermedicalscience. 2014;8:485.
- Reversible suppression of white blood cells (leukopenia) or platelets (thrombocytopenia)
- Elevated estrogen levels; use can produce galactorrhea (a milky nipple discharge) in women and gynecomastia (enlargement or swelling of breast tissue) in men
- Male sexual dysfunction which resolves with discontinued use
- Increased potential for bone marrow suppression (myelosuppression) when used together with drugs capable of causing bone marrow suppression
One elderly user experienced reversible mental confusion from cimetidine.12Marshall ME, Butler K, Hermansen D. Treatment of hormone-refractory stage D carcinoma of prostate with coumarin (1,2-benzopyrone) and cimetidine: a pilot study. Prostate. 1990;17(2):95-9.
Interactions with other treatments
Cimetidine may interact with several medications and therapies:13Pantziarka P, Bouche G, Meheus L, Sukhatme V, Sukhatme VP. Repurposing drugs in oncology (ReDO)—cimetidine as an anti-cancer agent. Ecancermedicalscience. 2014;8:485.
- Hydroxychloroquine: cimetidine may interact with the antimalarial medication hydroxychloroquine (another drug used off-label for some types of cancer).
- Methadone and opioids: cimetidine affects the metabolism of methadone, sometimes resulting in higher blood levels and a higher incidence of side effects. Cimetidine is also known to increase the effects of several opioids and can lead to extreme plasma levels of these drugs and a fatal overdose.
- Psychoactive medications: cimetidine can interact with a number of different psychoactive medications, including tricyclic antidepressants and selective serotonin reuptake inhibitors, causing increased blood levels of these drugs and subsequent toxicity.
Cimetidine can interfere with vitamin D metabolism, preventing an expected rise in serum levels and the benefits from that.14Kennedy DA, Cooley K et al. Vitamin D: pharmacokinetics and safety when used in conjunction with the pharmaceutical drugs used in cancer patients: a systematic review. Cancers (Basel). 2013 Mar 11;5(1):255-80. You might want to have your vitamin D levels checked before, during, and after using cimetidine. If your levels are low before starting cimetidine, talk with your doctor about using vitamin D supplements.
Do not use/contraindications
Due to possible elevated estrogen levels from use, people with estrogen-sensitive cancers, including some breast cancers and gynecologic cancers, should use H2 blockers with caution.15Chang R. Beyond the Magic Bullet: The Anti-Cancer Cocktail. New York: Square One Publishers. 2012; Mathes RW, Malone KE, Daling JR, Porter PL, Li CI. Relationship between histamine2-receptor antagonist medications and risk of invasive breast cancer. Cancer Epidemiology, Biomarkers & Prevention. 2008 Jan;17(1):67-72.
Due to suppression of white blood cells or platelets, use during chemotherapy should also be approached with caution.16Pantziarka P, Bouche G, Meheus L, Sukhatme V, Sukhatme VP. Repurposing drugs in oncology (ReDO)—cimetidine as an anti-cancer agent. Ecancermedicalscience. 2014;8:485.
References