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Antihistamines explained

Published on April 12, 2018

Antihistamines explained

What is an antihistamine?

Antihistamines are used by allergy sufferers to help relieve allergy symptoms and also to prevent allergy symptoms from occurring. Antihistamines are available over the counter without a prescription. Talk to your pharmacist to work out which antihistamine would be best for you.

What types of antihistamines are available?

Two types of antihistamines are used for allergies. They are known as first-generation antihistamines and second-generation antihistamines. First generation antihistamines commonly have the side effect of drowsiness whereas second generation antihistamines do not. This can affect your ability to operate machinery, drive your car and think clearly. Alcohol can also increase the drowsiness effect. Some people may use these medicines for the drowsiness effect, particularly where allergic symptoms such as runny or congested nose affects sleep. Second-generation antihistamines are less likely to cause the side effect of drowsiness.

Why are antihistamines used?

Antihistamines are used to assist with managing a number of allergic health conditions. Antihistamines can’t cure these conditions but they often provide much needed relief from symptoms. Some allergic conditions can have a huge impact on the quality of life for sufferers, particularly if they are chronic or recurring. Antihistamines are commonly used to help manage the symptoms of:

  • Hayfever – this common allergic condition can cause sneezing, runny nose, inflammation of nose and eyes, and itching of eyes, nose and throat.
  • Allergic rhinitis – allergic reaction to substances, which causes inflammation of the nose, for example dust mites.
  • Allergic skin conditions – such as eczema, rashes, hives (urticaria) and generalised itching (pruritus).
  • Allergic conjunctivitis – inflammation of the eyes, red, itchy and watery eyes.
  • Bites and stings – if there is an allergic reaction symptoms of raised circular weals that may or may not be itchy can appear.
  • Food allergies – for mild reactions of hives, runny nose, itchy nose and mouth. More severe reactions, known as anaphylaxis require emergency treatment.

How do antihistamines work?

As the name suggests, antihistamines work by stopping histamine affecting your body’s cells in the normal way. Histamine is a substance used by the body’s immune system to help protect the body from foreign substances. When histamine is released, this starts a cascade of inflammatory reactions such as swelling of small blood vessels. Histamine is generally very helpful, however in allergic reactions there may be too much histamine which leads to a number of symptoms such as runny nose, itchy eyes and swelling.

These symptoms are as a result of an over active immune system which has mistaken harmless substances as a threat. These substance are known as ‘allergens’ and commonly include pollen, dander, mould and dust.

Antihistamines target specific molecules in your cells, called receptors. They work by blocking these receptors in each cell so that histamine can’t reach its target and activate them. If the histamine can’t activate the cells like it usually would, it decreases your body’s reaction to allergens and therefore helps to suppress the irritating symptoms of allergy.

How do I choose an antihistamine?

Your doctor or pharmacist may advise a particular antihistamine depending on the cause of your allergy. Antihistamines taken constantly rather than intermittently are generally more effective. This is particularly true for hayfever, during times when the pollen count is high and regular contact with the allergen is unavoidable. Taking the medication regularly in this case will help keep symptoms under control.

For more information, visit the ASCIA website.

Reference: 1. Merck Manual Consumer Version (Accessed 6 march 2018)

CHCANZ.CFEX.18.04.0413

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