Antihistamine: Medical Information & Health Facts

What Are Antihistamines?

An antihistamine is a medication that blocks the action of histamine, a chemical released by the immune system during allergic reactions. When your body encounters allergens like pollen, dust mites, or certain foods, it produces histamine as a protective mechanism, leading to symptoms such as sneezing, itching, swelling, and a runny nose.

These medications block histamine receptors in your body, reducing or preventing allergy symptoms such as sneezing and itching. According to the NHS, this blocking action makes antihistamine drugs essential tools for managing various conditions, such as seasonal hay fever, hives, and conjunctivitis. Antihistamines are available in multiple forms, including pills, chewable tablets, capsules, liquids, nasal sprays, and eye drops.

How Do Antihistamines Work?

The mechanism by which antihistamine drugs function involves a complex biochemical process within your body. When you encounter an allergen, your immune system identifies it as a threat and releases histamine from specialised cells called mast cells and basophils. This histamine then travels through your bloodstream, seeking to bind with histamine receptors located throughout your body, especially in the nose, eyes, throat, lungs, stomach, and skin.

Antihistamine medications work by occupying these histamine receptors before histamine can bind to them. This blocking action prevents the typical allergic response cascade, including inflammation, increased mucus production, smooth muscle contraction, and blood vessel dilation.

There are different types of histamine receptors in your body, primarily H1 and H2 receptors. Most antihistamine tablets target H1 receptors, which are responsible for classic allergy symptoms. Some specialised antihistamine drugs target H2 receptors, which are mainly found in your stomach and help regulate acid production. This targeted approach allows doctors to recommend specific antihistamine medications tailored to your symptoms and needs.

Common Types of Antihistamines

These medications are typically classified into different generations based on their development timeline and characteristics:

H1 Receptor Antihistamines: H1 antihistamines block histamine at H1 receptors, which are located on smooth muscles, endothelial cells, and the central nervous system. This prevents the classic allergic responses such as vasodilation, increased vascular permeability, and smooth muscle contraction.

Examples include:

• Diphenhydramine
• Chlorpheniramine
• Hydroxyzine
• Promethazine
• Doxylamine

H2 Receptor Antihistamines: H2 antihistamines block H2 receptors located in the stomach lining (parietal cells). This reduces gastric acid secretion and helps manage acid-related disorders.

Examples include:

• Diphenhydramine
• Chlorpheniramine
• Hydroxyzine
• Promethazine
• Doxylamine

Uses of Antihistamines

Here are some of the uses of H1-antihistamines:

• Allergic rhinitis: Used to relieve symptoms such as sneezing, runny nose, nasal congestion, and itchy or watery eyes by blocking the effects of histamine.
• Urticaria (hives): Used to reduce redness, swelling, and itching caused by histamine release in the skin, providing effective symptomatic relief.
• Anaphylaxis: Administered as part of anaphylaxis management to counteract itching, hives, and swelling, while epinephrine addresses life-threatening reactions.
• Atopic dermatitis: Helps relieve itching associated with atopic dermatitis, reducing scratching and improving patient comfort.
• Allergic conjunctivitis: Used to alleviate red, itchy, and watery eyes resulting from histamine release in the conjunctiva.
• Motion sickness and nausea: Certain H1-antihistamines prevent and treat motion sickness, vertigo, and nausea by acting on histamine receptors in the central nervous system.

Here are some of the uses of H2-antihistamines:

• Peptic ulcer disease: Reduce gastric acid secretion, promoting healing of gastric and duodenal ulcers.
• Gastro-oesophageal reflux disease (GERD): Decrease stomach acid production, relieving heartburn, regurgitation, and oesophageal irritation.
• Zollinger-Ellison syndrome: Control excessive gastric acid secretion caused by gastrin-producing tumours, preventing complications such as ulcers.
• Prevention of stress-induced ulcers: Lower gastric acid secretion in critically ill patients, reducing the risk of stress-related gastrointestinal bleeding.

Side Effects of Antihistamines

Common side effects of first-generation antihistamines include:

• Drowsiness and sedation: First-generation antihistamines cross the blood-brain barrier, causing significant sleepiness and reduced alertness.
• Dry mouth and throat: These medications have anticholinergic effects, which reduce saliva and mucus production.
• Blurred vision: Anticholinergic action can interfere with the eyes’ ability to focus, resulting in temporary blurred vision.
• Dizziness and lightheadedness: Mild drops in blood pressure or central nervous system effects may lead to dizziness or lightheadedness.
• Difficulty concentrating: Cognitive impairment, including difficulty focusing, is more common in older adults.
• Urinary retention: These drugs can worsen urinary retention, especially in men with an enlarged prostate.
• Constipation: Slowed intestinal motility due to anticholinergic properties can cause constipation.
• Increased appetite and weight gain: Some patients may experience increased appetite and gradual weight gain with long-term use.

Typical side effects of second-generation antihistamines include:

• Headache: May occur due to minor changes in blood vessel tone or histamine-related pathways.
• Fatigue: Can happen in sensitive individuals, particularly with higher doses or prolonged use.
• Dry mouth: Rarely significant but can increase the risk of dental problems if persistent.
• Nausea or upset stomach: Usually mild and can be minimised by taking the medication with food.
• Dizziness: Occasionally occurs in some patients and may affect balance or coordination temporarily.

Precautions and Warnings with Antihistamines

• Drowsiness and impaired alertness: Some antihistamines, particularly first-generation types, can cause sedation. Patients should avoid driving, operating machinery, or performing tasks that require full mental alertness.
• Alcohol and CNS depressants: Concomitant use of antihistamines with alcohol or sedative medications can enhance drowsiness and central nervous system depression.
• Elderly patients: Older adults are more susceptible to side effects such as confusion, urinary retention, constipation, and dizziness.
• Pregnancy and breastfeeding: Antihistamines should be used during pregnancy or lactation only under medical supervision, as some drugs can affect the foetus or infant.
• Glaucoma and urinary retention: Patients with narrow-angle glaucoma or prostate enlargement should use antihistamines cautiously due to their anticholinergic effects.
• Renal and hepatic impairment: Dose adjustments may be necessary in patients with kidney or liver dysfunction to avoid drug accumulation and toxicity.
• Drug interactions: Certain antihistamines, especially cimetidine, can interfere with liver enzymes, affecting the metabolism of other medications and potentially increasing their toxicity.
• Children: Infants and young children may be more sensitive to sedative or paradoxical excitatory effects, such as restlessness or irritability.
• Allergic reactions: Though rare, hypersensitivity reactions, including rash, swelling, or angioedema, may occur.
• Long-term use: Prolonged use should be medically supervised, especially in older adults or those with hepatic or renal impairment, to monitor for cumulative side effects or drug tolerance.

Antihistamines for Children

Paediatric use of antihistamine tablets requires special consideration regarding dosing, formulation, and age-appropriate options. Most second-generation antihistamine drugs are considered safer for children because they cause less drowsiness and have fewer cognitive effects. However, some antihistamine tablets are not recommended for children under certain ages, particularly those under two years old.

When considering antihistamine treatment for your child, consult with your paediatrician about the most appropriate option. They can recommend specific formulations and dosing schedules and monitor for any adverse effects. Many liquid antihistamine medications are available for younger children who cannot swallow tablets, ensuring accurate dosing and easier administration.

Are Antihistamines Safe During Pregnancy?

Antihistamines can be used during pregnancy, but their safety depends on the specific drug and the stage of pregnancy. Some second-generation H1-antihistamines, such as loratadine and cetirizine, are generally considered safe for short-term use in pregnancy (particularly in the second and third trimesters) under medical supervision, to relieve allergy symptoms like sneezing, a runny nose, and itching.

First-generation H1-antihistamines, such as diphenhydramine, may also be used if needed, but they can cause drowsiness and should be taken with caution.

H2-receptor antagonists such as famotidine may be used during pregnancy to manage heartburn or gastro-oesophageal reflux and are generally regarded as safe under medical supervision. Despite this, all antihistamines should be used during pregnancy only under the guidance of a doctor, as some drugs may have potential risks in the first trimester or may affect the foetus if taken inappropriately.

Drug Interactions with Antihistamines

• Alcohol and sedatives: These combinations dramatically increase the risk of profound drowsiness, impaired coordination, and potentially dangerous respiratory depression, particularly with first-generation antihistamine medications.
• Antidepressants: Monoamine oxidase inhibitors (MAOIs) and tricyclic antidepressants can enhance antihistamine side effects, including dry mouth, blurred vision, confusion, and urinary retention.
• Anticholinergic medications: When combined with antihistamine drugs, these medications create additive effects that increase risks of constipation, urinary retention, blurred vision, and cognitive impairment.
• Antipsychotic medications: These combinations may increase sedation levels and raise the risk of dangerously low blood pressure, particularly in elderly patients or those with existing cardiovascular conditions.
• Antibiotics and antifungals: Certain medications in these categories can alter how your body processes antihistamine tablets, potentially leading to toxicity or reduced effectiveness of allergy treatment.

Over-the-Counter vs. Prescription Antihistamines

Common over-the-counter antihistamines include diphenhydramine (first-generation) and loratadine or cetirizine (second-generation). They provide effective relief for mild to moderate allergy symptoms when used as directed. They are widely available at pharmacies and are generally safe when used as directed for short-term relief.

Prescription antihistamine drugs may be stronger, longer-acting, or specially formulated for specific conditions. These are often recommended when over-the-counter options prove ineffective or inappropriate for your particular situation. Prescription antihistamines might include combination products targeting multiple symptoms simultaneously or medications designed for chronic conditions like severe urticaria or persistent allergic rhinitis.

Consider symptom severity, duration of treatment needed, potential side effects, and any underlying health conditions. While over-the-counter options work well for occasional seasonal allergies, persistent or severe symptoms may require prescription alternatives that offer better symptom control with fewer side effects.

Conclusion

Understanding antihistamine drugs and the differences between first- and second-generation options empowers you to make informed decisions about allergy management and overall health. Whether you're dealing with seasonal allergies, chronic hives, or occasional allergic reactions, antihistamine tablets can significantly improve your quality of life when used appropriately.

Doctors can help determine which antihistamine works best for your specific situation, considering factors such as your symptoms, lifestyle, other medications, and overall health status.

At Metropolis Healthcare, we understand the importance of comprehensive health monitoring when managing allergies and medication effects. Our extensive network of over 4,600 service centres and 10,000+ touchpoints across India ensures convenient access to essential diagnostic services that can help identify underlying causes of allergic reactions and monitor your health while using antihistamine medications. Through our convenient home sample collection service, you can access specialized allergy testing and routine health monitoring from the comfort of your home, supporting informed decisions about your antihistamine use and overall health management.

FAQs

Can antihistamines help with hay fever?

Yes, antihistamine medications are commonly used to treat hay fever symptoms such as sneezing, itchy and watery eyes, and a runny nose effectively. They work by blocking histamine, the chemical your body releases during allergic reactions to pollen and other environmental allergens.

Can antihistamines cause drowsiness?

Some antihistamine drugs, especially first-generation types like diphenhydramine, can cause significant drowsiness and sedation. Second-generation antihistamine tablets are formulated to cause less sleepiness and are often preferred for daytime use when alertness is essential.

What is the best time to take antihistamines?

The optimal timing depends on the specific antihistamine type you're using. Non-drowsy second-generation antihistamine medications can be taken during the day, while sedating first-generation antihistamine tablets are best taken at night if drowsiness concerns you.

Are there natural alternatives to antihistamines?

Natural substances like quercetin, butterbur (Petasites hybridus), and vitamin C have shown limited antihistamine-like effects in small studies; however, they are not substitutes for medical antihistamines and should only be used after consulting a healthcare provider, They are not substitutes for standard antihistamine treatment and should be used only after consulting a qualified healthcare provider

Can antihistamines be used for anxiety?

Certain first-generation antihistamines, such as hydroxyzine, may be used short-term for anxiety symptoms due to their sedative properties, but they are not standard treatments for anxiety disorders. Certain first-generation antihistamines, such as hydroxyzine, may be used short-term for anxiety symptoms due to their sedative properties, but they are not first-line treatments and should be used only under medical supervision.

Can you take antihistamines with alcohol?

You should avoid drinking alcohol while taking any antihistamine medication, especially first-generation types. Combining alcohol with antihistamine tablets increases sedation, impairs judgement significantly, and raises the risk of accidents or serious adverse effects.

What are the side effects of antihistamines?

• Drowsiness (especially first-generation types)
• Dry mouth and throat
• Dizziness and lightheadedness
• Headache
• Nausea or stomach upset
• Blurred vision
• Constipation

What are the signs of an antihistamine overdose?

• Severe drowsiness or difficulty staying awake
• Confusion and disorientation
• Rapid or irregular heartbeat
• Seizures or convulsions
• Difficulty breathing
• Loss of consciousness

References

1. https://my.clevelandclinic.org/health/treatments/antihistamines
2. https://medlineplus.gov/ency/patientinstructions/000549.htm
3. https://www.nhs.uk/medicines/antihistamines/
4. https://www.ncbi.nlm.nih.gov/books/NBK538188/
5. https://www.healthdirect.gov.au/antihistamines