From the Guidelines
H2 blockers are not recommended as first-line treatment for anaphylaxis, and their use should not delay the administration of epinephrine, which is the primary life-saving intervention. The primary treatment for anaphylaxis is epinephrine (adrenaline), which should be administered immediately as an intramuscular injection, typically 0.3-0.5 mg for adults or 0.15 mg for children using an auto-injector or from an ampule 1. H2 blockers like ranitidine or famotidine may be used as adjunctive therapy after epinephrine administration, usually alongside H1 antihistamines. When used, typical doses are famotidine 20 mg IV or ranitidine 50 mg IV for adults. The rationale for adding H2 blockers is that anaphylaxis involves both H1 and H2 receptors, and blocking both receptor types may help control symptoms like hypotension and flushing more effectively than H1 blockers alone. However, the certainty of evidence for the benefit of H2 blockers in anaphylaxis is very low, and additional well-designed controlled trials are needed to further inform this practice 1.
Some key points to consider when treating anaphylaxis include:
- Administering epinephrine as the first-line pharmacotherapy for uniphasic and/or biphasic anaphylaxis 1
- Not delaying the administration of epinephrine for anaphylaxis 1
- Monitoring patients for at least 4-6 hours due to the risk of biphasic reactions, and those with severe reactions should be observed for 24 hours 1
- Considering antihistamines and/or glucocorticoids as secondary treatment, but not as a reliable intervention to prevent biphasic anaphylaxis 1
- Educating patients about anaphylaxis, risk of recurrence, trigger avoidance, self-injectable epinephrine, and thresholds for further care 1
It is essential to prioritize the administration of epinephrine and not delay it for any reason, including the use of H2 blockers or other adjunctive therapies 1. The use of H2 blockers should be considered on a case-by-case basis, taking into account the individual patient's needs and the potential benefits and risks of their use.
From the Research
Effectiveness of H2 Blockers in Treating Anaphylaxis
- There is limited evidence to support the use of H2 blockers as a primary treatment for anaphylaxis 2, 3.
- Epinephrine is widely recognized as the first-line treatment for anaphylaxis, and its use is strongly recommended by guidelines 4, 5, 3, 6.
- H2 blockers, along with H1 antihistamines and glucocorticoids, are sometimes used as adjunctive treatments, but their effectiveness in anaphylaxis is not well established 2, 3.
- One study suggests that antihistamines, including H1 and H2 blockers, do not act as rapidly as epinephrine and may not relieve all symptoms of anaphylaxis 3.
- Another study found that patients treated with prehospital antihistamines were less likely to have uncontrolled reactions and be admitted after the reaction, but this study did not specifically examine the effectiveness of H2 blockers 6.
Comparison with Other Treatments
- Epinephrine is the most effective treatment for anaphylaxis, with a rapid onset of action and the ability to antagonize multiple mediators involved in the reaction 5, 3.
- H1 antihistamines and glucocorticoids may be used as adjunctive treatments, but their role in anaphylaxis management is not well defined 2, 3, 6.
- Corticosteroids, in particular, may not be beneficial in the treatment of anaphylaxis, and their use should be revisited 6.
Clinical Implications
- The use of H2 blockers as a primary treatment for anaphylaxis is not supported by the available evidence 2, 3.
- Epinephrine should be administered promptly in cases of anaphylaxis, and its use should be prioritized over other treatments 4, 5, 3, 6.
- Further research is needed to establish the effectiveness of H2 blockers and other adjunctive treatments in anaphylaxis management 2, 3, 6.