H2 Blockers: Names and Characteristics
The main H2 (histamine-2) receptor antagonists available for clinical use are cimetidine, ranitidine, famotidine, and nizatidine. 1
Available H2 Blockers and Their Properties
Cimetidine: An imidazole derivative that was one of the first H2 blockers developed. It contains N"-cyano-N-methyl-N'-[2-[[(5-methyl-1H-imidazol-4-yl)methyl]thio]-ethyl]guanidine as its active ingredient. 2
Ranitidine: A furan-based H2 blocker that is approximately 7 times more potent than cimetidine with a longer duration of action. 1, 3
Famotidine: A guanidino-thiazole H2 blocker that is the most potent of the group, approximately 8 times more potent than ranitidine and 40 times more potent than cimetidine. 4, 5
Nizatidine: A thiazole derivative with similar potency to ranitidine but with more complete absorption. 6, 7
Pharmacological Differences
Potency ranking (from most to least potent): famotidine > nizatidine ≈ ranitidine > cimetidine 5, 8
Duration of action: Famotidine has a longer duration of action compared to ranitidine and cimetidine. 5
Drug interactions: Cimetidine interacts with the cytochrome P-450 hepatic enzyme system, while famotidine, ranitidine, and nizatidine have minimal or no interaction with this system. 5, 7
Clinical Applications
All H2 blockers are effective for treating:
H2 blockers work better as prophylactic treatment rather than acute treatment for histamine-mediated conditions, as they prevent histamine from binding to receptors. 1
Formulations and Dosing
Cimetidine: Available in doses of 200 mg, 300 mg, 400 mg, and 800 mg tablets; typical dosing is 30-40 mg/kg/day divided in 4 doses (for pediatric patients) 1, 2
Ranitidine: Available as peppermint-flavored syrup and effervescent tablets; typical dosing is 5-10 mg/kg/day divided in 2-3 doses 1
Famotidine: Available in 20 mg and 40 mg tablets; typical dosing is 1 mg/kg/day divided in 2 doses 1, 4
Nizatidine: Available in 150 mg and 300 mg capsules; typical dosing is 10 mg/kg/day divided in 2 doses 1, 6
Important Clinical Considerations
H2 blockers can develop tachyphylaxis (reduced effectiveness) within 6 weeks of initiation, limiting their potential for long-term use. 1, 11
H2 blockers are generally well-tolerated with an overall incidence of side effects in the range of 2-3%. 5, 8
H2 blockers with anticholinergic effects (particularly cimetidine) can be associated with cognitive decline, especially in elderly populations. 1
When treating conditions like mast cell activation syndrome, combining H1 and H2 receptor antagonists provides more comprehensive symptom control than either agent alone. 10
Unlike proton pump inhibitors, famotidine does not interfere with the antiplatelet activity of clopidogrel, making it a potential alternative in patients on dual antiplatelet therapy. 11