What is the recommended IV dose of Piriton (Chlorpheniramine) for a patient experiencing a severe allergic reaction, considering factors such as age, weight, and underlying medical conditions like impaired renal or liver function?

IV Chlorphenamine (Piriton) Dosing for Severe Allergic Reactions

For adults and children over 12 years, administer 10 mg IV slowly; for children 6-12 years give 5 mg; for children 6 months to 6 years give 2.5 mg; and for infants under 6 months give 250 µg/kg. 1

Critical First-Line Treatment Requirement

Before discussing chlorphenamine dosing, epinephrine is the only first-line treatment for anaphylaxis and must be administered immediately - chlorphenamine is strictly adjunctive therapy and should never delay or replace epinephrine administration. 1 Intramuscular epinephrine 0.01 mg/kg (maximum 0.5 mg) in the anterolateral thigh must be given first for any severe allergic reaction. 1

Standard IV Chlorphenamine Dosing by Age

The dosing is administered either intramuscularly or intravenously slowly: 1

• Adults and children >12 years: 10 mg IV slowly
• Children 6-12 years: 5 mg IV slowly
• Children 6 months to 6 years: 2.5 mg IV slowly
• Infants <6 months: 250 µg/kg IV slowly

These doses come from the Association of Anaesthetists of Great Britain and Ireland guidelines for suspected anaphylactic reactions associated with anaesthesia, which provide the most specific IV dosing recommendations. 1

Administration Technique

Administer chlorphenamine slowly via IV push to minimize adverse effects. 1 The medication should be given after or concurrent with epinephrine administration, not as a substitute for it. 1

Role in Anaphylaxis Management

Chlorphenamine provides only symptomatic relief of itching and urticaria - it does not relieve stridor, shortness of breath, wheezing, gastrointestinal symptoms, or shock. 1 Very limited scientific evidence supports H1-antihistamines in emergency anaphylaxis treatment, and they work too slowly to address life-threatening symptoms. 1 The maximum oral/IV dose of diphenhydramine (a comparable H1-antihistamine) is 50 mg with dosing of 1-2 mg/kg, which provides context for similar agents. 1

Adjunctive Medications to Consider

Beyond chlorphenamine, consider adding: 1

• H2-antihistamine: Ranitidine 1-2 mg/kg (maximum 75-150 mg) IV or famotidine 1-2 mg/kg (maximum 75-150 mg) IV 1, 2
• Corticosteroids: Hydrocortisone 200 mg IV (adults >12 years), 100 mg (6-12 years), 50 mg (6 months-6 years), or 25 mg (<6 months); alternatively methylprednisolone 1 mg/kg (maximum 60-80 mg) IV 1
• Bronchodilators: Albuterol nebulized (3 mL adults, 1.5 mL children) for bronchospasm not responsive to epinephrine 1

Special Considerations for Renal/Hepatic Impairment

The provided guidelines do not specify dose adjustments for renal or hepatic impairment for chlorphenamine in acute anaphylaxis. In emergency situations, standard age-based dosing should be used without delay, as the immediate life-threatening nature of anaphylaxis outweighs theoretical concerns about drug accumulation. 1

Critical Pitfalls to Avoid

• Never substitute chlorphenamine for epinephrine - this is the most common fatal error in anaphylaxis management 1, 3
• Do not delay epinephrine administration to give antihistamines first - failure to administer epinephrine early has been repeatedly implicated in anaphylaxis fatalities 1, 3
• Recognize that 35-72% of anaphylaxis patients require more than one dose of epinephrine, so have additional doses ready even after giving chlorphenamine 4
• Be aware that chlorphenamine itself can rarely cause anaphylaxis - if symptoms worsen after administration, consider this paradoxical reaction 5

Repeat Dosing and Duration

Chlorphenamine can be continued every 6 hours for 2-3 days post-discharge as part of adjunctive therapy to prevent biphasic reactions. 1 However, the primary focus during acute management is ensuring adequate epinephrine administration and hemodynamic support with IV fluids (large volumes of normal saline for persistent hypotension). 1