One of the most common problems that occurs in the management of anaphylaxis is that oral sedating antihistamines are given when in fact they are contraindicated.

The ASCIA Guidelines (updated May 2026): Acute Management of Anaphylaxis states:

Antihistamines:

• Have no role in treating or preventing respiratory or cardiovascular symptoms of anaphylaxis.

• Do not use oral sedating antihistamines as side effects (drowsiness or lethargy) may mimic some signs of anaphylaxis. Injectable promethazine should not be used in anaphylaxis as it can worsen hypotension and cause muscle necrosis.

• Non-sedating antihistamines may be used to help alleviate persistent cutaneous symptoms once anaphylaxis has been treated. ¹

The reasoning

The reason is that cardiogenic shock and hypoxia in anaphylaxis can cause drowsiness, confusion and lethargy, and these can also be caused by sedating antihistamines. This can cause a diagnostic dilemma if the patient exhibits any of these signs post administration of a sedating antihistamine.

Also, there are over 20 mediators of inflammation at play in anaphylaxis of which histamine is only one, so antihistamines are ineffective in managing anaphylaxis.

The problem of injectable promethazine causing hypotension is particularly important as hypotension can be a major cause of morbidity and mortality in anaphylaxis and anything that worsens it is clearly to be avoided. Muscle necrosis is also a major complication.

Even if the anaphylaxis has resolved, there is still a case for not giving sedating antihistamines until after the observation period has expired, as in up to 20% of cases the patient may experience a biphasic response (the recurrence of anaphylaxis without any further exposure to the original trigger) and sedating antihistamines may again create a diagnostic dilemma.

Once anaphylaxis has been treated then the use of non-sedating antihistamines is acceptable to treat “persistent cutaneous symptoms”

Concerns from the Acute Anaphylaxis Clinical Care Standard

The Australian Commission on Safety and Quality in Health Care has released the Acute Anaphylaxis Clinical Care Standard ²

On page 8 the Commission states

“Studies continue to show high rates of corticosteroid and antihistamine administration for the initial treatment of anaphylaxis. ^13-15 This is of concern as delayed administration of adrenaline is a risk factor for fatal anaphylaxis". ^16-17 

My own clinical approach

In my own clinical practice, I approach all allergic reactions that have the potential to develop into anaphylaxis as anaphylaxis until proven otherwise. This is because I do not wish to assume a simple allergic reaction is occurring, treat with a sedating antihistamine and then have anaphylaxis declaring itself later on. I find patients are happy to endure the discomfort of itching once they understand what I am doing.

Join my training to learn how to effectively recognise and manage anaphylaxis using current Australian guidelines. Click here to Learn more.

Dr Sue Flanagan  

14th June 2026 

References 

1. ASCIA Guidelines, Acute Management of Anaphylaxis (Updated May 2026);  https://www.allergy.org.au/hp/anaphylaxis/acute-management-guidelines 

1. Australian Commission on Safety and Quality in Health Care Acute Anaphylaxis Clinical Care Standard Nov 2021