Antihistamines: Everything you need to know

If you're one of the 18% of Australians who experiences allergic rhinitis (hay fever),¹ you're probably familiar with the mid-spring, itchy-eyed dash to the pharmacy for an antihistamine.

But are you buying the right one? And what, exactly, do antihistamines do?

As the name suggests, antihistamines work by blocking the chemical histamine, which is produced by your immune system to help protect your body against foreign allergens.

Typically, your body overreacts to a harmless substance that you're sensitive to, like pollen, grass, animal hair, mould or dust mites. When histamine is released, it causes an inflammation response, which can lead to symptoms like:

• sneezing
• runny or blocked nose
• itchy or watery eyes
• throat irritation
• blocked nose and snoring.

By blocking histamine, antihistamine drugs can help relieve these symptoms.

Antihistamines come in different varieties, including tablets, sprays, ointments and eye drops. They're usually designed to treat hay fever, as well as dust mite allergies, allergic conjunctivitis, hives, and allergic reactions to things like insect bites.

There are 2 main types of antihistamine tablets at the pharmacy. These are sedating (first generation), designed for night-time use, and non-sedating (second generation), which are for the day.

They're usually available over the counter. Some common varieties include:²

Sedating:

• diphenhydramine
• promethazine
• dexchlorpheniramine
• doxylamine.

Non-sedating:

• desloratadine
• bilastine
• fexofenadine
• cetirizine
• loratadine.

Deciding on which antihistamine is right for you, or whether you should take one at all, will come down to your health background and the severity of your symptoms.

It's also important to consider potential side effects before taking an antihistamine.

As sedating antihistamines cause drowsiness, they're usually recommended for night-time use to help you sleep. Less-common side effects of sedating antihistamines include:

• blurred vision
• constipation
• dry mouth.

Even though non-sedating antihistamines usually advertise themselves as 'non-drowsy', drowsiness may occur. Other less common side effects of non-sedating antihistamines also include:

• headaches
• insomnia
• fatigue
• rashes.

You should talk to your doctor or pharmacist before taking antihistamines if you:

• are pregnant or breastfeeding
• have issues with your prostate
• live with kidney or liver disease
• plan on giving the antihistamine to a child under 6.²

Often, finding the right antihistamine comes down to trial and error. Talk to your doctor or pharmacist about your symptoms, and take note of how different antihistamines affect you.

If you've taken any antihistamine, it's important to be careful when operating machinery (including cars and bicycles). Always follow the advice on the label.

While antihistamines are a common go-to for quick allergy relief, there are alternative or complementary treatments available.

Making certain lifestyle adjustments, particularly during periods when you're most sensitive to an allergen, is often the first-line response to an allergic reaction. And for those with mild symptoms, this may be all that's required.

Adjustments may involve things like avoiding trigger areas (like parks), keeping windows closed and regularly washing your hands and face on days when the pollen count is high.

Alternative treatments will often come down to your allergy and the severity of your symptoms. Some common ones include:

When allergy symptoms are particularly severe around your nose, your doctor may recommend a low-dose steroid spray. This treatment requires a prescription, and some are designed for ongoing use.

Designed for short-term use, decongestants are usually recommended when antihistamines and corticosteroid nasal sprays haven't worked. They work by reducing congestion and swelling in your nasal passage, and they typically come as tablets, syrups or nasal sprays.

A saline (salt-based) water solution can be flushed through your sinuses with a special squeezable pump that's available at the pharmacy. The flush is designed to remove allergens in your nasal passage, such as pollen and dust.

For more severe and ongoing allergies, allergy immunotherapy may be recommended. This treatment is administered either via regular injections (subcutaneous) or with daily tablets, sprays or drops under the tongue (sublingual).

Allergy immunotherapy works by slowly exposing you to something you're allergic to, with the hope that, over time, you'll become desensitised to it. This means that the treatment can take years, often up to 3. However, if successful, allergy immunotherapy can significantly reduce or even eliminate allergy symptoms for a number of years or, in some cases, permanently.

If allergies are affecting your health and wellbeing, talk to your doctor about your options. You may need to have your allergy tested.

The Australasian Society of Clinical Immunology and Allergy (ASCIA) offers information and support for Australians and New Zealanders living with allergies, providing medical practice, training, education and research.

Allergy and Anaphylaxis Australia has resources available online. Visit their website or call them for help and support on 1300 728 000.