Chlorphenamine maleate

How it works

It competitively antagonises histamine at H1 receptors, reducing histamine-mediated vasodilatation, increased capillary permeability and pruritus; its lipophilicity allows CNS penetration, producing sedation.

Prescribing in practice

• In anaphylaxis it is an adjunct only and must never delay or replace intramuscular adrenaline, which is the first-line life-saving treatment.
• It has marked sedative and antimuscarinic effects, so caution is needed in the elderly, in prostatic hypertrophy, glaucoma and where alertness must be maintained.
• The intravenous route should be given by slow injection, as rapid administration can cause transient hypotension and CNS stimulation.

Monitoring

Monitor for excessive sedation, anticholinergic effects and, after parenteral use, blood pressure and conscious level.

Counselling the patient

• This medicine commonly causes drowsiness; avoid driving or operating machinery if affected.
• Alcohol and other sedatives will increase the drowsiness.
• Seek urgent help if allergic symptoms return or worsen after treatment.

Evidence & guidelines

Use as an adjunct in anaphylaxis after adrenaline is supported by UK Resuscitation Council guidance.