Understanding antihistamines: what they are and how they work
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If you’ve ever reached for allergy medicine to ease sneezing, itching, or runny nose, you’ve likely encountered antihistamines. These medications are a cornerstone of allergy management, offering relief from symptoms that can disrupt daily life. But what exactly is an antihistamine, and how does it work? Whether you’re exploring options like levocetirizine or curious about the differences between first and second generation antihistamines, this article will give you the information to help make more informed choices—we’ll break down the basics of antihistamines, how they function, and how these medicines have evolved over time.
What is an antihistamine?
An antihistamine is an allergy medicine designed to counteract the effects of histamine, a chemical your body releases during an allergic reaction. Histamine plays a role in causing symptoms like sneezing, watery eyes, and a runny nose, as it helps your immune system respond to perceived threats. However, in people with allergies, the immune system can overreact to harmless substances like pollen, pet dander, or dust mites, leading to uncomfortable symptoms.
By blocking histamine receptors, antihistamines help reduce or prevent these reactions, offering relief from allergy symptoms. They’re commonly available in various forms, including tablets, liquids, and nasal sprays, making them a versatile option for managing allergies.1
How do antihistamines work?
To understand how antihistamines work, it’s helpful to know that histamine binds to specific receptors in your body, triggering allergy symptoms. Antihistamines block these receptors, preventing histamine from attaching and causing a reaction.
There are two main types of histamine receptors: H1 and H2. Most allergy medications target H1 receptors, which are responsible for symptoms like sneezing, runny nose, and itchy eyes.
When you take antihistamine allergy medicine, it doesn’t stop your body from producing histamine—it simply prevents it from causing symptoms.2
How does histamine cause allergy symptoms?
When your body encounters an allergen, it triggers a chain reaction that results in familiar allergy symptoms. Here is a simple breakdown of how, and why, histamine causes allergy symptoms:
- Histamine is normally released when the body detects something harmful, like an infection. It causes blood vessels to expand to help protect your body from the threat.
- Sometimes the body encounters something it mistakes for a threat, like pollen or dust, and releases histamine.
- Histamine binds to receptors in the body, leading to common allergy symptoms like a runny nose, sneezing, and itchy, watery eyes.
- Antihistamine allergy tablets are designed to block histamine from attaching to these receptors, which can help reduce the severity of symptoms.3
What allergies can antihistamines treat?
Antihistamines are a widely recognized class of medications, but their applications extend beyond what many might assume. While they are most commonly associated with seasonal allergies, their mechanism of action allows them to address a broader range of histamine-related symptoms. The U.S. Food and Drug Administration (FDA) has approved H1 antihistamines to help manage symptoms tied to several conditions, including:
- Allergic rhinitis, commonly known as hay fever
- Allergic conjunctivitis, which affects the eyes
- Hives and other allergic skin conditions
- Sinusitis
- Angioedema, or swelling under the skin
- Motion sickness
- Nausea and vomiting1
Because these symptoms can stem from different causes, it is always a good idea to consult your doctor or pharmacist before using allergy tablets. It’s also worth noting that antihistamines do not help to treat the symptoms of cold and flu, which often overlap with allergy symptoms.
First-generation vs. second-generation antihistamines
Antihistamines are commonly categorized into two main generations: first-generation and second-generation. Each group has distinct characteristics that influence how it works in the body and the potential side effects it might cause.
First-generation antihistamines
First-generation antihistamines, such as diphenhydramine, are an older class of medications. While they are often effective at relieving allergy symptoms, they are also known for causing significant drowsiness. This sedative antihistamine side effect occurs because these drugs can readily cross the blood-brain barrier, a protective filter that separates the brain from the bloodstream. Once they enter the central nervous system, they can affect brain function, which commonly results in sleepiness. Because of this, first-generation antihistamines are often not the preferred choice for daytime use when staying alert is important.2
Second-generation antihistamines
In response to the sedative effects of earlier drugs, a newer class of less-drowsy allergy medicine, known as second-generation antihistamines, was developed. These are designed to be safer and more effective, with a much lower likelihood of causing drowsiness.2
This is because they are less able to cross the blood-brain barrier, which allows them to relieve allergy symptoms without significantly affecting brain function. Their design also provides longer, more sustained relief, making them a preferred choice for managing daily allergy symptoms. Levocetirizine is one of the more recently approved antihistamines in this class, providing fast-acting, long-lasting, allergy relief with minimal side effects.4
Why Xyzal uses levocetirizine
Levocetirizine is the active ingredient in Xyzal, and it represents a precise approach to allergy relief. It is the pharmacologically active component of a prior-approved antihistamine, cetirizine.5
Research indicates that levocetirizine has a high affinity for the histamine H1-receptor, which is the target for allergy medications. This means it is designed to work efficiently at the source of symptoms.4
How long does an antihistamine take to work?
The time it takes for an antihistamine to work can vary depending on the specific medication and how it’s administered. Xyzal® Allergy 24, for example, starts relieving symptoms in as little as 45 minutes.
FAQs about antihistamines
Antihistamines are designed to block the effects of a substance called histamine, which your body can release when it mistakes something harmless, like pollen or dust, for a threat. This release can cause symptoms like itchy eyes, a runny nose, and sneezing. Taking an antihistamine when you start experiencing allergy symptoms, can help reduce their severity.3
When it comes to first generation vs second generation antihistamines, the difference comes down to their chemical structure and how they interact with the body: First-generation antihistamines can easily cross the blood-brain barrier, a protective filter for your brain. This is what can lead to side effects like drowsiness. In contrast, second-generation, allergy medicines are designed differently. They do not cross the blood-brain barrier as readily, which is why they are less likely to cause drowsiness or interact with other medications.2
For many people, taking an antihistamine daily may be an effective way to manage persistent allergy symptoms. Whether this approach is right for you often depends on the type and severity of your allergies. For personalized advice on managing your allergy symptoms, it is always best to consult a healthcare professional.1
Whether or not an antihistamine is more effective if taken at night depends on the type used. Always check the label and use antihistamines as directed. Xyzal® Allergy 24 is designed to be taken at night, it relieves allergies while you sleep so you wake up refreshed and ready for the next day.
Sources:
- Cleveland Clinic. Antihistamines: types, uses and side effects [Internet]. Cleveland (OH): Cleveland Clinic; 2024 Jul 26 [cited 2026 May 27]. Available from: https://my.clevelandclinic.org/health/treatments/antihistamines
- Church MK, Church DS. Pharmacology of antihistamines. Indian J Dermatol [Internet]. 2013 May-Jun;58(3):219-224 [cited 2026 May 27]. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC3667286/
- National Health Service (NHS). Antihistamines [Internet]. London: NHS; 2023 May 16 [cited 2026 May 27]. Available from: https://www.nhs.uk/medicines/antihistamines/
- Walsh GM. The anti-inflammatory effects of levocetirizine: are they clinically relevant or just an interesting additional effect? Allergy Asthma Clin Immunol [Internet]. 2009;5(1):14 [cited 2026 May 27]. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC2804563/
- MedChemExpress. Levocetirizine dihydrochloride [Internet]. Monmouth Junction (NJ): MedChemExpress; [cited 2026 May 27]. Available from: https://www.medchemexpress.com/levocetirizine-dihydrochloride.html?srsltid=AfmBOors-hq5x2HXcsgU3MA6XMo3-BK404GNpr0QCu1Prj8DaSy5T9-
- Asthma and Allergy Foundation of America (AAFA). Can I develop tolerance to antihistamines? [Internet]. Arlington (VA): Asthma and Allergy Foundation of America; 2022 Aug 15 [cited 2026 May 27]. Available from: https://community.aafa.org/db/ask-the-allergist/record/can-i-develop-tolerance-to-antihistamines
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Xyzal® relieves tough allergy symptoms such as sneezing, itchy eyes, watery eyes, runny nose, itchy nose, and itchy throat. Take it at night for 24 hours of continuous allergy relief that won’t fade into the next day.
Xyzal® Allergy 24HR
Take Xyzal at night for 24 hours of continuous adult allergy relief that won't fade. Powerful allergy relief that's just as effective at hour 24 as at hour 1.
