Antihistamines encompass a family of drugs that work by blocking the release of histamine in the body. 

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Histamine and Allergy

Histamine is utilised for multiple different functions including in our immune system to facilitate inflammation and in our brain where it promotes the feeling of wakefulness.

The release of histamine is a core feature of most allergic reactions and is responsible for producing symptoms such as weeping of the skin, itchiness, difficulty breathing, diarrhoea and even vomiting. You may have come across them due to their use in the treatment of allergy related conditions such as hay fever (allergic rhinosinusitis). In hay fever pollen allergens trigger histamine release in the nasal passage leading to swelling of the lining of the nose, sneezing and nasal secretion/discharge. 

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Types of Antihistamine

Antihistamines are used to block the unwanted effects of histamine release, with different types of anti-histamine used depending on which histamine action is needed to be blocked. There are 2 types of histamine receptor in the body H1 and H2. H1 receptors are involved in allergy related symptoms while H2 receptors cause stomach acid production. 

Given that histamine plays such varied roles in the body it is important to not block all histamine release but instead to use specific antihistamines that preferentially act on specific histamine functions in the body. 

Antihistamines used for the management of allergy related symptoms block the H1 receptor. This class is further subcategorised into first and second generation antihistamines. First generation antihistamines easily cross the blood brain barrier, and suppress the feeling of wakefulness causing drowsiness, hence these drugs can be used to treat conditions like insomnia. Second generation anti-histamines do not cross the blood brain barrier and only work on peripheral histamine receptors hence they are often referred to as non-drowsy antihistamines and are preferred for the treatment of allergic conditions. 

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H1

First Generation

• Chlorphenamine (Piriton)
• Hydroxyzine
• Cyclizine
• Promethazine
• Diphenhydramine (Benadryl)

Second Generation

• Cetirizine
• Fexodenadine
• Loratidine (Claritin)
• Levocetirizine

H2

• Ranitidine
• Famotidine

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Antihistamines and Eczema/Atopic Dermatitis

Extensive research has been undertaken into the role of histamine in atopic dermatitis (AD), though itch is caused by histamine release in allergic reactions, histamine does not drive itch in atopic dermatitis. This has been supported by multiple studies which have not shown any improvement in itch despite regular antihistamine use. Currently, the main use of antihistamines in AD is for it’s sedating benefit which can aid sleep during periods of intense itch. Longterm use of sedating anti-histamines is not recommended as they reduce sleep quality and can lead to daytime drowsiness. Other unwanted side effects of antihistamines can include tummy aches, dry mouth and eyes, blurry vision and headaches.

In summary antihistamines play a very limited role in atopic dermatitis, they can be used briefly to aid sleep during intense periods of itch due to their ability to cause drowsiness but do not help with reducing itch (despite often being prescribed for this purpose). 

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