Woman sneezing on her handkerchief

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. Do not underestimate the drowsiness effect of first-generation antihistamines. 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.1

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 infection. The histamine then causes small blood vessels to swell and expand, also known as inflammation. Histamine is generally very useful, however in allergic reactions there may be too much histamine and its effects may need to be blocked.

In allergic conditions, the immune system has become overactive and mistakes harmless substances as a threat.2 These substance are known as ‘allergens’ and commonly include pollen, dander, mould and dust. For most people, the immune reaction to these harmless substances is normal and appropriate. However, for people with allergic conditions, the reaction is excessive and the body produces too much histamine. This results in excessive inflammation and the typical symptoms of allergy such as sneezing, swelling, rash, runny nose and itching. For example, in people with the allergic condition hayfever, contact with pollen in the nose, throat and eyes triggers much more histamine than normal which produces the associated symptoms of itching, sneezing, runny eyes etc.3

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.2

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.3

For more information, visit Telfast’s Break Through page.