What Is An Antihistamine For Allergies?

Allergies affect millions worldwide. They can manifest as itchy eyes and a runny nose or as severe and potentially life-threatening. One of the most common ways to manage allergy symptoms is through antihistamines.

What is an antihistamine and what does an antihistamine do?

We’re answering these questions and exploring how they pair with other allergy treatments like immunotherapy. Learn the positives and negatives of antihistamines and whether or not they’re right for managing your allergy symptoms.

What Is an Antihistamine?

Antihistamines are a class of drugs that help alleviate allergy symptoms like hay fever, itching, and hives by countering the effects of histamine, a natural chemical produced by your body in response to allergens. As the name suggests, antihistamines work against histamine's effects and can provide relief from symptoms.

While they are effective in providing short-term symptomatic relief, antihistamines do not address the root cause of allergies and may have some negative side effects with prolonged use.

What Does an Antihistamine Do?

Antihistamines function by binding to histamine receptors, preventing histamine from attaching to these sites and producing an allergic response. When histamine is blocked from binding to its receptors, it doesn’t trigger the responses responsible for symptoms like itching, sneezing, nasal congestion, or swollen eyes from allergies.

Not all antihistamines treat the same symptoms. Antihistamines are classified into two main categories based on the type of histamine receptor they primarily target:

1. H1 antihistamines
2. H2 antihistamines

1. H1 Antihistamines

H1 antihistamines are the more common type and are used to block the effects of histamine at H1 receptors. These receptors are found in various tissues, including the nasal and respiratory passages, skin, and blood vessels. H1 antihistamines are used to relieve allergy symptoms, like itching, sneezing, runny nose, and hives.

2. H2 Antihistamines

H2 antihistamines target H2 receptors, primarily found in the stomach's lining. They are used to reduce stomach acid production and are often prescribed to manage conditions like acid reflux, peptic ulcers, and gastroesophageal reflux disease (GERD). These medications help alleviate symptoms associated with excessive stomach acid, such as heartburn and indigestion.

It's important to note that the terms "H1" and "H2" refer to specific types of histamine receptors, and antihistamines designed to block these receptors have different applications.

Types of Antihistamines for Allergies

Diving deeper into the question, “What is an antihistamine for allergies?” you’ll learn they are broken down into two categories: first-generation and second-generation. Each group has distinct characteristics, including how drowsy they make you, how long their effects last, and any side effects you may have.

First-Generation H1 Antihistamines

First-generation H1 antihistamines are the older class of antihistamine drugs. They were among the first antihistamines developed and have been widely used for many years. While they’re very effective at relieving allergy symptoms, the most important difference in this generation of antihistamines is that they cross the blood-brain barrier causing allergy fatigue and drowsiness.

Examples of first-generation H1 antihistamines include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), and doxylamine. Common characteristics of this type of antihistamine are drowsiness, impaired coordination, and a shorter duration of action that requires more frequent dosing.

Second-Generation H1 Antihistamines

Second-generation H1 antihistamines are newer drugs (first coming to market in the 1980s) designed to provide effective allergy symptom relief without the sedative side effects of their first-generation counterparts. They are a significant improvement in antihistamine allergy treatment and less likely to cross the blood-brain barrier but may still pose a risk when taken long-term(1).

Examples of second-generation antihistamines are drugs like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). They’re less likely to cause drowsiness, have a longer duration of action, and are generally preferred for their improved side effect profile.

Second-generation antihistamines are generally preferred for their improved side effect profile, making them a first choice for individuals seeking immediate allergy symptom relief without the drowsiness. However, the choice between first and second-generation antihistamines should be based on individual needs, allergist guidance, and how it pairs with other immunotherapy treatments you’re undergoing like prescription allergy drops.

Potential Antihistamine Side Effects

While they are generally safe and effective when used as directed, what is an antihistamine if not a medication? As with any medication, this means they come with potential side effects depending on how it interacts with your body. It's crucial to be aware of these potential side effects and use antihistamines under the guidance of your allergist.

Common Antihistamine Side Effects

• Sedation: First-generation antihistamines make you drowsy and sleepy. Don’t drive or operate heavy machinery when on these medications.
• Dry mouth: Both first-generation and second-generation antihistamines can cause dry mouth, known as xerostomia. Stay hydrated and use sugar-free gum or lozenges to alleviate this side effect.
• Blurred vision: Some antihistamines, especially first-generation ones, can lead to blurred vision. This affects your ability to focus and may pose risks in situations requiring clear eyesight.
• Urinary retention: Using antihistamines can result in urination difficulty or urinary retention, which can be concerning for individuals with underlying bladder issues.
• Gastrointestinal issues: While less common, antihistamines can occasionally cause stomach discomfort, constipation, or diarrhea.
• Cardiac arrhythmia: While rare, the development of arrhythmias has been noted in various case reports(2).

Given these potential risks, it is essential to use antihistamines as directed and consult an allergist. They will determine an allergy treatment plan for long-term effectiveness.

Can You Combine Antihistamines and Immunotherapy?

Allergy immunotherapy is a treatment option for people with severe allergies. It involves gradually exposing the patient to increasing amounts of the allergen to desensitize their immune system over time. This method is particularly effective if you have allergies that do not respond well to other treatments or medications.

While antihistamines and immunotherapy serve the same purpose – managing allergy symptoms – they work through different mechanisms. Antihistamines provide quick relief from symptoms, but they do not modify the underlying allergic response. On the other hand, immunotherapy aims to alter the immune system's response to the allergen in the long term.

In many cases, antihistamines can be used in conjunction with immunotherapy. In one study, premedication with antihistamines was shown to potentially enhance the efficacy of specific-allergen immunotherapy(3).

Antihistamines may be prescribed to manage symptoms during the initial stages of immunotherapy when the allergen exposure is at its highest. Over time, as the immune system becomes less reactive to the allergen due to immunotherapy, the need for antihistamines may decrease.

Positives and Negatives of Antihistamines

Antihistamines play a crucial role in managing allergy symptoms, providing relief from discomfort and helping lead a more comfortable life. However, like any medication, they come with both positive and negative aspects. We’ve grouped the advantages and disadvantages of using antihistamines for allergies to supplement your understanding of these medications.

Positives of Antihistamines

• Quick relief: Antihistamines offer rapid relief from allergy symptoms, often within minutes to an hour of ingestion.
• Over-the-counter accessibility (OTC): Many antihistamines are available without a prescription.
• Improved second-generation antihistamines: Modern second-generation antihistamines are less likely to cause drowsiness and other side effects compared to their first-generation counterparts.

Negatives of Antihistamines

• Temporary relief: Antihistamines provide symptomatic relief but do not address the root cause of allergies. Once the medication wears off, symptoms will return.
• Potential side effects: Some antihistamines, particularly first-generation ones, can cause side effects such as drowsiness, dry mouth, and blurred vision.
• Tolerance: With prolonged use, individuals may develop tolerance to antihistamines, requiring higher doses to achieve the same level of relief.
• Dependency: First-generation antihistamines, due to their sedative effects, can lead to dependency in some users who rely on them for sleep. This can result in difficulty sleeping without the medication.
• Limited efficacy: While antihistamines are effective for many types of allergies, they may not provide adequate relief for severe or complex allergies.
• Secondary treatment: These are not a first-line treatment for allergic rhinitis

So what is an antihistamine and what does an antihistamine do? Widely used as an option for managing allergy symptoms, antihistamines for allergies work by blocking the effects of histamine and providing relief from itching, sneezing, and other allergy symptoms. They may be used alongside allergen immunotherapy but it's important to understand their limitations and potential side effects.

Additional Sources

1. https://www.ncbi.nlm.nih.gov/books/NBK547896/
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1874280/
3. https://pubmed.ncbi.nlm.nih.gov/11149995/