Can Benadryl (diphenhydramine) be administered intramuscularly (IM)?

Intramuscular Administration of Diphenhydramine

Yes, Benadryl (diphenhydramine) can be administered intramuscularly (IM) as confirmed by FDA labeling, which specifically indicates it is approved for both intravenous and intramuscular administration. 1

Dosing Guidelines for IM Administration

Adults:

• Standard IM dose: 10-50 mg deep intramuscularly 1
• For allergic reactions/anaphylaxis: 25-50 mg (1-2 mg/kg) 2, 3
• Maximum daily dosage: 400 mg 1

Pediatric Patients:

• 5 mg/kg/24 hr or 150 mg/m²/24 hr divided into four doses 1
• Maximum daily dosage: 300 mg 1

Administration Technique

When administering diphenhydramine IM:

• Inject deep into large muscle mass (vastus lateralis or ventrogluteal site preferred)
• Inspect solution visually for particulate matter and discoloration prior to administration 1
• Use proper IM injection technique to minimize complications 4
• For adults, use appropriate needle length to ensure medication reaches muscle tissue rather than subcutaneous fat

Clinical Applications

IM diphenhydramine is particularly useful in:

1. Anaphylaxis management: As part of treatment protocol alongside epinephrine 2

   • Combined with H2 antagonists (like ranitidine) for superior effect 2

2. Acute allergic reactions: When oral route is impractical 1

3. Treatment of dystonic reactions: 1-2 mg/kg (maximum initial dose: 50 mg) 3, 5

Precautions and Monitoring

• Monitor for sedation and respiratory depression, especially when combined with other sedatives 3
• Be aware of potential for paradoxical excitation or agitation with rapid administration 3
• Consider cardiac monitoring in patients with heart disease due to potential QT interval prolongation 6
• Exercise caution in elderly patients or those with hepatic impairment 3

Alternative Considerations

• For long-term management of allergic conditions, second-generation antihistamines may be preferable due to fewer side effects and less sedation 7
• In hospice settings where IV access is limited, subcutaneous administration has been documented as safe in terminally ill patients 8

Important Caveats

• Avoid administering too rapidly to prevent adverse effects
• Be aware that diphenhydramine dependence through IM route has been reported, though rare 6
• Consider patient-specific factors such as age, weight, and comorbidities when determining appropriate dosing

IM diphenhydramine remains a valuable option when oral administration is not feasible or when rapid antihistamine effect is needed, particularly in emergency situations.