The Core of Allergy Relief: How Antihistamines Work
When your body encounters an allergen, like pollen or pet dander, your immune system releases a chemical called histamine [1.3.2]. Histamine is responsible for the classic allergy symptoms: sneezing, itching, runny nose, and watery eyes [1.4.1]. Antihistamines work by competitively blocking histamine from attaching to specific receptors on cells, known as H1-receptors [1.4.1]. By preventing histamine from binding, these medications suppress the inflammatory response and alleviate the uncomfortable symptoms associated with allergies [1.4.6].
There are two primary classes of antihistamines that target different receptors in the body [1.4.2]:
- H1-antihistamines: These are used to treat allergy symptoms [1.4.1].
- H2-antihistamines: These are used for gastrointestinal issues like acid reflux by reducing stomach acid secretion [1.4.5].
This article focuses on the H1-antihistamines commonly used for allergies.
Active Ingredients: The Generational Divide
The primary active ingredients in antihistamines are categorized into "generations," mainly based on their chemical structure and side effect profile, particularly drowsiness [1.2.3].
First-Generation Antihistamines Introduced in the 1940s, first-generation antihistamines are effective but are known for causing drowsiness [1.3.8]. This is because they readily cross the blood-brain barrier and affect histamine receptors in the central nervous system [1.3.3, 1.4.2]. Their effects typically last for 4 to 6 hours [1.4.2].
Common first-generation active ingredients include:
- Diphenhydramine: The active ingredient in Benadryl and many over-the-counter sleep aids like ZzzQuil [1.2.2, 1.2.5].
- Chlorpheniramine: Found in products like Chlor-Trimeton [1.2.1].
- Doxylamine: A common ingredient in sleep aids like Unisom [1.2.5].
- Hydroxyzine: A prescription antihistamine often used for itching and anxiety [1.2.5].
Second- and Third-Generation Antihistamines Developed in the 1980s, second-generation antihistamines were designed to be more selective for peripheral H1 receptors and not cross the blood-brain barrier as easily [1.3.8, 1.4.2]. This makes them significantly less likely to cause sedation [1.3.3]. They also offer the convenience of longer-lasting relief, often up to 24 hours from a single dose [1.7.3]. Third-generation antihistamines are derived from second-generation ones and are also non-drowsy [1.3.2].
Common second-generation active ingredients include:
- Loratadine: The active ingredient in Claritin [1.2.6].
- Cetirizine: The active ingredient in Zyrtec [1.2.6].
- Fexofenadine: The active ingredient in Allegra [1.2.7].
- Desloratadine: The active ingredient in the prescription medication Clarinex [1.2.1].
- Levocetirizine: The active ingredient in Xyzal [1.4.4].
Comparison of Antihistamine Generations
| Feature | First-Generation (e.g., Diphenhydramine) | Second-Generation (e.g., Loratadine, Cetirizine) |
|---|---|---|
| Drowsiness | High likelihood of sedation [1.3.7] | Low to no sedation (non-drowsy) [1.3.4] |
| Duration | Shorter-acting (4-6 hours) [1.4.2] | Longer-acting (12-24 hours) [1.4.2] |
| Primary Use | Allergies, insomnia, motion sickness [1.4.1] | Allergic rhinitis (hay fever), hives [1.3.2] |
| Side Effects | Dry mouth, dizziness, impaired coordination [1.3.1, 1.7.7] | Fewer side effects; mild headache or stomach pain possible [1.2.6] |
| Brain Permeability | Easily crosses blood-brain barrier [1.3.3] | Does not cross blood-brain barrier significantly [1.7.1] |
Beyond the Active: Inactive Ingredients
While the active ingredient does the heavy lifting, antihistamine medications also contain numerous inactive ingredients, or excipients. These components are added during manufacturing to serve various purposes [1.5.4]. Although rare, some individuals can have allergic reactions to these supposedly "inactive" substances [1.5.4].
Common inactive ingredients include:
- Fillers: To add bulk to a tablet, such as lactose or corn starch [1.5.1, 1.5.5].
- Binders: To hold the tablet together, like povidone [1.5.1, 1.5.5].
- Coatings: For easier swallowing and to control release, often containing hydroxypropyl methylcellulose or polyethylene glycol (PEG) [1.5.5].
- Dyes and Flavorings: To provide color and taste, especially in liquids and chewable tablets [1.5.4]. Common dyes include Brilliant Blue and Sunset Yellow FCF [1.5.1].
- Preservatives: To prevent microbial growth [1.5.4].
Nearly all oral medications contain at least one potentially reactive excipient, such as lactose, PEG, or various dyes [1.5.3]. For example, the inactive ingredients in Zyrtec tablets include lactose, corn starch, povidone, and polyethylene glycol [1.5.5].
Conclusion
Antihistamines are composed of one or more active ingredients—classified into generations based on their side effects—and a host of inactive ingredients that give the medication its form and stability. The active compounds, such as diphenhydramine or loratadine, work by blocking the effects of histamine in the body to relieve allergy symptoms [1.4.5]. Understanding the difference between drowsy first-generation and non-drowsy second-generation options allows consumers to make informed choices based on their needs and daily activities [1.3.4]. Always read the label to identify the active ingredients and consult a healthcare provider to determine the best option for you [1.7.3].
For more detailed information on antihistamines, you can visit the NHS page on Antihistamines.
