What are Antihistamines and How Do They Work?

When you have an allergic reaction, your immune system produces high quantities of chemicals known as histamines¹. These can cause typical allergy symptoms such as coughing, sneezing, itching, blocked noses and/or watery eyes¹.

Antihistamines are a group of medicines that help prevent and treat these common allergy symptoms, and make life with allergies that little bit easier².

ZYRTEC® products typically contain a type of antihistamine called cetirizine – but there are many types, and each is slightly different².

What Does An Antihistamine Do?

Antihistamines help to block some of the adverse effects caused by the histamines our bodies produce, in response to an allergen entering our systems².

They may be available with a prescription, or as an over-the-counter (OTC) medication.

Taking an antihistamine can help relieve a number of common allergy symptoms, including²:

Congestion
Coughing
Itchy or watery eyes*
Dry or red eyes
Itchy skin or hives
Skin rashes
Itching of the nose or throat*
A runny or blocked nose*
Sneezing*

Types of Antihistamines

Antihistamines can be divided into two major subtypes²:

H1 receptor antagonists or H1 blockers – these are typically used to treat symptoms of allergies, insect bites, hypersensitivity to other medication and hives
H2 receptor antagonists or H2 blockers – these are generally used to treat gastrointestinal conditions, including acid reflux, peptic ulcers, gastritis, motion sickness, heartburn, nausea, and vomiting.

Antihistamines for Allergies

When it comes to allergy treatments, you'll most likely be taking an H1 antihistamine blocker². ZYRTEC® products, for instance, contain cetirizine, which is an H1 blocker.

Other examples of H1 antihistamines for allergies include²:

Azelastine
Fexofenadine
Diphenhydramine
Loratadine

How Do Antihistamines Work?

When you come into contact with an allergen such as pollen, mold, animal dander or dust, your body produces a chemical called histamine. Antihistamines reduce the effects of histamine on our body by blocking the receptors in our brains that usually react to it² ³.

In a typical immune response, your body will produce histamine to help fight infection. The chemical causes your blood vessels to expand and your skin to swell to protect your body from what it views as ‘foreign’ substances³.

With an allergic response, however, this process is triggered by something harmless that your body mistakes for a threat.

How to Choose the Right Antihistamine for Your Needs

Choosing the best antihistamine for your allergies will depend on several different factors, including:

The symptoms you want to treat
Your preferred form of medication (e.g. tablets or nasal sprays)
Whether you want a non-drowsy formula

Remember, some antihistamines will take longer to enter your system than others. So, you'll also need to consider how quickly you need relief when choosing your antihistamine.

To help you find the best antihistamine for you, we've created a comparison page where you can compare different antihistamine products and find one that works for you.

See which antihistamine is right for you.

FAQs

How Long Does ZYRTEC® Take to Work?

ZYRTEC® products start working at hour 1 after you first use them⁴. They are available in tablet and liquid gel form – as well as Zyrtec-D, which also contains a powerful decongestant.

How Long Does ZYRTEC® last?

When you use a ZYRTEC® product, you will experience 24 hours of relief from allergy symptoms⁴.

References

* Symptoms that are treated by ZYRTEC® or ZYTEC®-D