Antihistamines - Antihistamines are one of the most commonly used classes of medications for allergy relief and cough suppression. These drugs help alleviate symptoms like itching, runny nose, sneezing, and watery eyes by blocking the effects of histamine — a natural chemical released during allergic reactions.
Among them, Diphenhydramine (commonly known as Benadryl) is a first-generation antihistamine with both antiallergic and sedative properties. Other agents like Dextromethorphan and Benzonate are frequently used for cough control.
Antihistamines play an important role in managing conditions like seasonal allergies, common cold, urticaria (hives), and insomnia.
Classification
Pharmacologic Class (P): EthanolaminesExamples:
Diphenhydramine (Benadryl) – Antihistamine & SedativeAvailable Routes: Oral (PO), Intramuscular (IM), and Intravenous (IV).
Mechanism of Action (MOA)
Antihistamines work by blocking the H₁-histamine receptors in the body.
Histamine is released from mast cells during allergic reactions and binds to H₁ receptors, causing vasodilation, itching, increased mucus secretion, and inflammation. By preventing this binding, antihistamines effectively:
1. Reduce allergy symptoms such as itching, sneezing, and nasal congestion.In simple terms, antihistamines act like a “histamine blocker,” calming allergic reactions and soothing airway irritation.
Uses of Antihistamines
Antihistamines have a wide range of therapeutic applications, both for allergies and respiratory conditions.
1. Allergy Management:
- Relieve symptoms of seasonal allergies (hay fever).
- Treat allergic rhinitis, hives (urticaria), and itching.
2. Cough Suppression:
- Diphenhydramine and Dextromethorphan help reduce cough frequency by acting on the brain’s cough center.
- Benzonate numbs throat receptors, reducing the cough reflex.
3. Common Cold Relief:
Reduces sneezing, runny nose, and watery eyes.4. Motion Sickness & Nausea:
Some first-generation antihistamines prevent nausea and vomiting caused by travel sickness.5. Insomnia (Short-Term):
Because of their sedative properties, drugs like Diphenhydramine are used to aid sleep.Adverse Effects
While effective, antihistamines may cause several side effects, especially first-generation agents like Diphenhydramine, which cross the blood-brain barrier and cause sedation.
You can remember the major adverse effects with the mnemonic “COUGH”:
C – Confusion: Especially in older adults due to CNS depression.Additional Effects:
- Dry mouth
- Blurred vision
- Constipation
- Drowsiness and impaired coordination
Drug Interactions
Antihistamines interact with several other drugs, especially those that depress the central nervous system (CNS).
| Interacting Drug/Class | Effect | Clinical Implication |
|---|---|---|
| CNS Depressants (e.g., alcohol, sedatives, opioids) | ↑ Sedation | Increased drowsiness and risk of accidents |
| Monoamine Oxidase Inhibitors (MAOIs) | ↑ Anticholinergic effects | Dry mouth, constipation, urinary retention |
| Other Anticholinergics | Additive effects | Enhanced side effects like blurred vision or confusion |
Tip: Avoid alcohol and sedatives while taking antihistamines.
Contraindications
Antihistamines should be avoided or used cautiously in the following conditions:
Glaucoma: Risk of increased intraocular pressure.Precautions
- Avoid driving or operating machinery due to drowsiness.
- Do not combine with alcohol or sleep medications.
- Use lower doses in older adults and patients with liver disease.
- Encourage hydration to reduce dry mouth.
- For cough preparations, use short-term only (≤7 days) unless prescribed.
Clinical Tip for Students
From the image visual:
“Diphenhydramine (Benadryl) is used to suppress cough and reduce allergy symptoms.”
This key line simplifies the dual action of Diphenhydramine — both antiallergic and antitussive — making it a useful reference drug in exams and practical applications.
Summary Table
| Parameter | Details |
|---|---|
| Drug Class | Ethanolamines (Antihistamines) |
| Examples | Diphenhydramine, Dextromethorphan, Benzonate |
| Mechanism | Blocks H₁-histamine receptors → ↓ Itching, Cough, and Inflammation |
| Uses | Allergies, Cough, Motion sickness, Insomnia |
| Common Adverse Effects | Confusion, Drowsiness, GI upset, Hypotension |
| Interactions | CNS depressants, MAOIs |
| Contraindications | Glaucoma, BPH, Hyperthyroidism |
| Routes | PO, IM, IV |
Difference Between First and Second-Generation Antihistamines
| Feature | First Generation (e.g., Diphenhydramine) | Second Generation (e.g., Cetirizine, Loratadine) |
|---|---|---|
| Sedation | High | Minimal |
| Blood-Brain Barrier | Crosses easily | Poor penetration |
| Duration of Action | Short (4–6 hours) | Long (24 hours) |
| Anticholinergic Effects | Present | Absent or minimal |
| Common Use | Acute allergy, cough | Long-term allergy relief |
Key Takeaways
- Antihistamines block histamine receptors to relieve allergy and cold symptoms.
- Diphenhydramine (Benadryl) is a first-generation antihistamine with sedative and cough-suppressing properties.
- Avoid use with CNS depressants and MAOIs due to increased sedation and anticholinergic effects.
- Use cautiously in patients with glaucoma, BPH, or hyperthyroidism.
- Second-generation antihistamines are preferred for long-term use due to fewer side effects.
FAQs About Antihistamines
Q1. What are antihistamines used for?
They are used to treat allergic reactions, runny nose, itching, hives, and cough caused by allergies or upper respiratory infections.
Q2. What is the mechanism of Diphenhydramine?
It blocks histamine H₁ receptors, reducing inflammation, cough, and allergic symptoms.
Q3. Can antihistamines make you sleepy?
Yes. First-generation antihistamines like Diphenhydramine can cause drowsiness and sedation.
Q4. Is Diphenhydramine safe for children?
It can be used in children under medical supervision but may cause paradoxical excitement in some cases.
Q5. Can antihistamines be taken with alcohol?
No. Alcohol increases CNS depression and drowsiness, which can be dangerous.
Q6. What’s the difference between Diphenhydramine and Cetirizine?
Diphenhydramine is a first-generation sedating antihistamine, while Cetirizine is a second-generation non-sedating antihistamine.
