Why is diphenhydramine (Benadryl) administered during anesthesia?

Why Benadryl is Used in Anesthesia

Diphenhydramine (Benadryl) is primarily used in anesthesia as an alternative local anesthetic agent for patients with documented allergies to traditional "caine" anesthetics (lidocaine, bupivacaine, etc.), not for its antihistamine properties. 1, 2

Primary Indication: Alternative Local Anesthetic

The main reason diphenhydramine appears in anesthetic practice is as a second-line local anesthetic when patients report allergies to lidocaine and its chemical analogues. This is supported by:

• Proven efficacy: Diphenhydramine 1% produces equivalent anesthesia to lidocaine 1% for dermal procedures, with onset times of 4-7.5 minutes 2
• Clinical validation: Multiple studies demonstrate adequate anesthesia for laceration repair, oral surgery, and dermatologic procedures 1, 3, 4
• Mechanism: As an antihistamine with anticholinergic properties, diphenhydramine blocks sodium channels and provides local anesthetic effects when infiltrated 5

Important Caveats for Local Anesthetic Use

Infiltration with diphenhydramine is significantly more painful than lidocaine (median pain score 55mm vs 12.5mm on visual analog scale) 2, 6. Additionally:

• Duration of anesthesia is shorter than lidocaine (sensation returns in 19/30 injections within 45 minutes vs only 3/30 for lidocaine) 6
• Safety in areas with poor collateral perfusion (digits, pinna, nose) remains unestablished 2
• Benzyl alcohol with epinephrine may be a superior alternative, causing less infiltration pain than diphenhydramine 6

NOT Recommended for Anaphylaxis Management

Importantly, diphenhydramine should NOT be routinely used during anesthesia for prophylaxis or treatment of anaphylactic reactions. The 2009 anesthesia guidelines explicitly state there is no evidence to support the use of H2-blocking drugs (or by extension, H1 antihistamines like diphenhydramine) in the immediate management of anaphylaxis 7. Recent evidence further supports this:

• The 2020 AAAAI guidelines discourage first-generation antihistamines in acute anaphylaxis or for preventing biphasic reactions 8
• Second-generation antihistamines have similar efficacy with fewer adverse effects 9
• Despite guidelines, 62% of ED cases still inappropriately receive diphenhydramine for anaphylaxis/urticaria 8

Safety Concerns in Anesthetic Practice

When diphenhydramine IS used, exercise caution with its atropine-like anticholinergic effects 5:

• Use cautiously in patients with bronchial asthma, increased intraocular pressure, hyperthyroidism, cardiovascular disease, or hypertension
• Additive CNS depression with other anesthetic agents and alcohol 5
• May cause paradoxical excitation in pediatric patients or hallucinations/convulsions in overdose 5
• Contraindicated in neonates and premature infants 5
• Rapid IV administration can cause life-threatening adverse effects 10

Clinical Algorithm for Use

When encountering a patient claiming "caine" allergy requiring local anesthesia:

1. First-line: Consider benzyl alcohol with epinephrine (less painful infiltration than diphenhydramine) 6
2. Second-line: Use diphenhydramine 1% (NOT 2%, as higher concentration shows no additional benefit) 2
3. Warn patient: Infiltration will be more painful than traditional local anesthetics
4. Avoid: Areas with poor collateral circulation until more safety data available
5. Post-procedure: Refer patient for formal allergy testing, as true "caine" allergies are extremely rare 7

The bottom line: Diphenhydramine's role in anesthesia is limited to serving as an alternative local anesthetic in the rare patient with documented local anesthetic allergy—it has no role in anaphylaxis management during anesthesia. 7, 2