How should pheniramine maleate (Avil) be diluted for intramuscular injection to deliver an adult dose of 10–25 milligrams or a pediatric dose of 0.25–0.5 milligram per kilogram, ensuring a total injection volume of about 1–2 milliliters?

Pheniramine (Avil) IM Administration: Dilution Not Required

Pheniramine maleate for intramuscular injection does not require dilution—it should be administered undiluted directly from the ampule. The standard commercial preparation is ready for immediate IM use.

Standard IM Dosing

Adult Dosing

• 10 mg IM is the standard adult dose for acute allergic reactions and anaphylaxis management 1
• Administer slowly over 2-3 minutes if giving IV to minimize injection site pain and seizure risk 2
• Maximum initial dose: 50 mg 2

Pediatric Dosing

• Age-based dosing is preferred over weight-based calculation for practical administration 1:

  • >12 years: 10 mg IM
  • 6-12 years: 5 mg IM
  • 6 months to 6 years: 2.5 mg IM
  • <6 months: 250 mcg/kg IM

• Alternative weight-based approach: 0.25-0.5 mg/kg IM with maximum 50 mg 2

Preparation and Administration Technique

No Dilution Required

• Commercial pheniramine ampules contain the drug in ready-to-inject concentration 1
• Draw up the required dose directly from the ampule into the syringe
• Do not add diluent unless specifically preparing for slow IV administration 2

IM Injection Site Selection

• Anterolateral thigh (vastus lateralis) is the preferred site for IM injections, providing rapid absorption and lower complication rates 3, 4, 5
• Use adequate needle length to reach muscle beneath subcutaneous tissue—this is critical in overweight patients 3, 5
• Alternative sites include deltoid (adults) or ventrogluteal muscle, though dorsogluteal is no longer recommended due to nerve injury risk 5

Volume Considerations

• Standard adult dose of 10 mg typically comes as 1 mL volume in commercial preparations
• Pediatric doses of 2.5-5 mg represent 0.25-0.5 mL volumes
• These volumes are appropriate for IM injection without further dilution 3, 4

Clinical Context and Role in Therapy

Position in Anaphylaxis Management

• Pheniramine is adjunctive therapy only—never use as monotherapy for anaphylaxis 2
• Epinephrine must be given first for anaphylactic reactions 1
• Pheniramine is administered as secondary management after epinephrine and fluid resuscitation 1
• It relieves urticaria and itching but does not treat bronchospasm, laryngeal edema, or hypotension 1

Timing of Administration

• Give pheniramine after initial stabilization with epinephrine (50 mcg IV boluses for adults or 0.01 mg/kg IM for children) 1
• Administer alongside hydrocortisone (200 mg IV for adults, 100 mg for 6-12 years, 50 mg for 6 months-6 years) 1

Critical Safety Considerations

Common Adverse Effects

• Sedation is the most frequent side effect and should be anticipated 2
• Respiratory depression may occur, particularly when combined with other sedatives 1, 2
• Paradoxical excitement or agitation can develop, especially in young children 2
• Hypotension may occur with rapid IV administration 2

Monitoring Requirements

• Monitor vital signs and oxygen saturation during and after administration 2
• Be prepared to provide respiratory support if apnea develops, especially when pheniramine is combined with benzodiazepines or opioids 1, 2
• Watch for paradoxical agitation in pediatric patients 2

Contraindications and Precautions

• Avoid in severe hepatic disease—sedative effects may precipitate hepatic coma 2
• Contraindicated in angle-closure glaucoma, prostatic hypertrophy, and urinary retention 2
• Do not combine with MAOIs or other anticholinergic medications due to additive toxicity 2
• Avoid during pregnancy, especially first trimester, and during lactation 2

Common Pitfalls to Avoid

Dilution Errors

• Do not dilute for IM use—this is unnecessary and may lead to dosing errors
• Only dilute if preparing for slow IV push administration (over 2-3 minutes) 2

Route Confusion

• IM route is preferred when IV access is not immediately available 2
• If giving IV, administer slowly over 2-3 minutes to prevent pain and seizures 2

Therapeutic Misunderstanding

• Never substitute pheniramine for epinephrine in anaphylaxis—it does not treat life-threatening symptoms 1, 2
• Pheniramine only addresses urticaria and pruritus, not airway compromise or shock 1

Overdose Recognition

• Overdose can cause antimuscarinic toxicity (dry mucous membranes, blurred vision, hallucinations), CNS excitation (irritability, seizures), rhabdomyolysis, and rarely ventricular arrhythmias 6
• Aggressive supportive care is required for overdose management 6