Chemistry in Everyday Life


Antihistamine is medication to cure allergic rhinitis and other allergies. Usually, people take antihistamines as a low-cost, generic over-the-counter drug that can relieve nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with minimal side effects.

Antihistamines are mostly for short-term therapy. Chronic allergies increase the possibility of health conditions that antihistamines may not cure, like asthma, sinusitis, and lower respiratory tract infections. Consultation with a medical professional is necessary for those who decide to use antihistamines for longer-term use.



What is Antihistamine?

Antihistamine is a class of drugs that block the physiological action of histamine. Histamine is an organic compound that is active in local immune responses and also functions as a neurotransmitter. It also plays a role in the inflammatory response and mediator of pruritus. Basophils produce histamine, and it is also present in nearby connected tissues. Histamine enhances the permeability of the white blood cells, which helps pathogens engage with damaged tissues.

Classification of Antihistamine

 H1-antihistamines and H2-antihistamines are the two main classes of antihistamines.

H1-antihistamines function by binding to the H1 receptor of histamine in mast cells, smooth muscles, endothelial cells in the body and in the brain tuberomammillary nucleus. Antihistamines targeting the H1-receptor of histamine are useful in treating allergic reactions in the nose (itching, runny nose, and sneezing). And it also helps treat insomnia, motion sickness or dizziness caused by complications in the inner ear.

H2-antihistamines bind to histamines H2 receptors in the upper gastrointestinal tract, mainly in the stomach. Antihistamines targeting the H2-receptor of histamine are helpful in treating gastric acid symptoms (e.g., peptic ulcers and acid reflux).

Types of Antihistamines

There are basically four types of antihistamines, and they are;

  • H1-antihistamines
  • H2-antihistamines
  • H3-antihistamines
  • H4-antihistamines


H1-antihistamines refers to compounds that suppress the function of the H1 receptor. Since the H1 receptor exhibit constitutive function, H1-antihistamines can either be neutral receptor antagonists or inverse agonists. Usually, histamine binds to the H1 receptor and activates the receptor functions; receptor antagonists react by binding to the receptor and preventing the receptor activation by histamine. In comparison, inverse agonists bind to the receptor and, inhibit the binding of histamine and reduce its constitutive activity, which is inverse of histamine. Most antihistamines are inverse agonists of the H1 receptor but previously thought to be antagonists.

H1-antihistamines are of two different generations based on their properties; First generation and second generation.


These are the earliest H1 antihistamine medications that are widely available at relatively cheap rates. They are helpful in preventing allergic symptoms but are usually moderate to highly potent muscarinic acetylcholine receptor (anticholinergic) antagonists. The patient’s response and the adverse drug reactions occurrences vary significantly between the classes and agents within the classes.


Second-generation H1-antihistamines are new drugs that are much more selective for peripheral H1 receptors than central nervous system H1 receptors and cholinergic receptors. This selectivity decreases the risk of adverse drug reactions significantly, such as sedation, while ensuring adequate relief of allergic conditions. The explanation for their peripheral selectivity is, many of these compounds are zwitterionic at physiological pH levels (around 7.4 pH). They are very polar, which indicates that they are less likely to cross the blood-brain barrier and function outside the central nervous system. However, a few second-generation antihistamines, specifical cetirizine, can interact with CNS psycho-active drugs like bupropion and benzodiazepines.


H2-antihistamines, such as H1-antihistamines, function as inverse agonists and neutral antagonists. They work on the H2 histamine receptor sites primarily in gastric mucosa parietal cells, which are part of the endogenous signalling pathway for gastric acid secretion. Usually, histamine acts on H2 to induce acid secretion; thus, medications that suppress H2 signalling decrease gastric acid secretion.

H2-antihistamines are among the first-line therapies used to treat gastrointestinal abnormalities including, peptic ulcers and gastroesophageal reflux disorder. There are several formulations available via the counter. Much of the side effects are due to cross-reactivity of unintended receptors. Cimetidine, for example, is notorious for antagonising high-dose androgen testosterone and DHT receptors.


H3-antihistamine is a classification of medicines used to suppress the activity of histamine in the H3 receptor. H3 receptors are usually present in the brain. They are inhibitory autoreceptors present on the histaminergic nerve terminals, that modifies the release of histamine. The release of histamine in the brain induces the secondary release of excitatory neurotransmitters such as glutamate and acetylcholine by stimulating the H1 receptors in the cerebral cortex. As a result, unlike sedative H1-antihistamines, H3-antihistamines have stimulating and cognitive-modulating effects.


H4-antihistamines block the activation of the H4 receptors.

Side effects of Antihistamines

Antihistamines are toxic in nature. Comparatively, some are more harmful than others.

  • Older groups known as the first generation of antihistamines are more toxic, such as Benadryl and Chlor-Trimeton. They cause dizziness in particular.
  • In the other hand, the newer generation of antihistamines is relatively less toxic, such as Allegra, Clarinex and Zyrtec. Dry mouth is the main side effect of antihistamines, hence avoiding overdosage as much as possible is a better option.
  • Anticholinergic symptoms, like dry mouth and eyes, urinary retention, tachycardia, tinnitus, mydriasis, dizziness (mainly by using first-generation antihistamines)
  • Decreased tear production
  • Urinary retention (trouble urinating)
  • Blurred vision,
  • Constipation

FAQs about Antihistamine

Q.1. List out a few of the substances, or allergens, that cause allergies?

Answer. The top eight most popular factors that can induce an allergic reaction in certain people include:

Food, Dust, Pollen, Pet dander, saliva or urine, Mould, Insect bites and stings, Latex, Certain medications/drugs.

Q.2. How many types of antihistamines are there and give a few examples.

Answer. There are four types of antihistamines, and they are:


Example include; H1 antagonists/inverse agonists; Acrivastine, Buclizine, Chlorodiphenhydramine, Diphenhydramine (Benadryl), Ebastine, Embramine, Fexofenadine (Allegra/Telfast), Hydroxyzine (Vistaril), Levocabastine (Livostin/Livocab), Meclizine, Olopatadine, Trazodone.


Examples include; Cimetidine, Famotidine, Lafutidine, Nizatidine, Ranitidine, Roxatidine, Tiotidine.


Examples of selective H3-antihistamines include Clobenpropit, ABT-239, Ciproxifan, Conessine, A-349,821, Thioperamide.


Examples: Thioperamide, JNJ 7777120, VUF-6002.

Q.3. What are the differences between first-generation and second-generation antihistamines?

Answer. As the name suggests, the first-generation Antihistamine was the first kind approved by the Food and Drug Administration (FDA). They began to be accepted in the United States in the 1930s and are still in use till date. They function with histamine receptors in the brain and spinal cord along with other forms of receptors. The most remarkable thing bout this generation is that they can cross the blood-brain barrier, which results in a side effect like sleepiness.

Second-generation antihistamines came into the market in the 1980s after the FDA approval. Antihistamines of the second generation do not penetrate the blood-brain barrier to the point that the first generation does. Thus they do not induce sleepiness at standard dosage levels. These are safer than first-generation antihistamines because they do not trigger dizziness and interact with fewer drugs.

Q.4. What are the dosage types available for antihistamines?

Answer. Antihistamines come in a variety of ways, including Liquids, Lotions, Syrups, Gels, Eye drops, Tablets, Nasal sprays, Creams, Capsules, Suppositories.

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