Antihistamine

What is Antihistamine? 

Antihistamines are a class of drugs that are used to cure the allergies triggered by the physiological action of the histamines. Histamines are the organic compounds that are involved in the local immune responses and act as a neurotransmitter. Histamines take part in the inflammatory response and act as a mediator of pruritus. Basophils produce histamines, and they are found in the connected nearby tissues. Histamines increase the permeability of the white blood cells, and this makes the pathogens to get more engaged with the infected tissues. Antihistamine examples are ranitidine, loratadine, meclizine, cetirizine, etc.

(Image to be added soon)


Antihistamine Drugs Classification

Antihistamines drugs can be classified into two types according to the H receptor-targeted: -

  • H1: They are generally used to treat allergic reactions as well as mast cell-mediated disorders. This category can be further divided into two classes: first-generation H1 antihistamines, which have a central effect and are used as sedatives; and second-generation H1 antihistamine which have a lesser central effect and are used as antiallergic drugs.

  • H2: They are used for gastric reflux disease as they help in reducing the production of stomach acid by reversibly blocking the H2 histamines receptors in the parietal cells of the gastric mucosa.


Example of Drugs and Antihistamine Uses According to Antihistamine Classification

  • H1 First-Generation: Meclizine, Clemastine, Hydroxyzine, Brompheniramine, Dimetindene, Doxylamine, etc. that are used as sedative agents, antiallergic agents, to cure motion sickness, and an antiemetic agent. These drugs have strong sedative action and anticholinergic side effects.

  • H1 Second-Generation: Loratadine, Cetirizine, levocetirizine, azelastine, fexofenadine, etc. that are used as antiallergic agents, or adjuvant treatment for anaphylactic shock. These drugs are non-sedative or mildly sedative.

  • H2: Ranitidine, Cimetidine, Famotidine, etc. that are used to reduce the production of stomach acid and are generally used as a second-line treatment or in combination with proton pump inhibitors (PPIs). 


Antihistamine Side-Effects

The severity and the frequency of the adverse effects of histamines vary according to the type of antihistamine. The H1 first-generation antihistamines are considered to be more harmful, and they usually cause drowsiness. H1 antihistamines cause anticholinergic effects like dry mouth and eyes, dizziness, urinary retention, mydriasis tinnitus, and tachycardia along with headaches. Benadryl and Chlor-Trimeton are examples of such drugs. The side effects of H2 antihistamines include confusion, dizziness, headaches. For example, Allegra, Clarinex, and Zyrtec. Cimetidine can lead to erectile dysfunction and gynecomastia because of its antiandrogenic effect. Cimetidine and ranitidine decrease the renal excretion of creatinine.  


Solved Questions

Question 1. What is the difference between Claritin and Clarinex?

Answer: Claritin is loratadine that is metabolized in the liver to produce Clarinex that is desloratadine.


Question 2.  How do antihistamines protect against allergies?

Answer: Antihistamines block the action of the histamines that gets triggered when our body comes in contact with components like pollen, dust mites, etc. These components make the histamines come in action to cause allergies like swelling of the nose, running nose and eyes, etc. Antihistamines block the course of action of histamines and protect us from allergies. 


Question 3. Can antihistamines be used to cure skin allergies?

Answer: Yes, antihistamines can be used to take care of insect bites and skin allergies. Zyrtec and Claritin are both best antihistamines to help you with your skin allergies.


Question 4. What is an allergy?

Answer: An allergy is an immunological hypersensitivity that is mediated by immunoglobulin E antibody (IgE). An allergy is not related to any disease or infections.


Question 5. What causes an allergy?

Answer: Allergies generally occur when your immune system reacts to a foreign substance like pollens, bee venom etc. Allergies are basically the reactions of your body coming into contact with foreign substances.

FAQ (Frequently Asked Questions)

1. Who Must Not Consume Antihistamines?

Though antihistamines can be safely consumed by the majority of people of all age groups, there are a few exceptions that must not consume antihistamines.

  • Women who are pregnant or are breastfeeding.

  • People with underlying health issues like epilepsy, high blood pressure, liver disease, kidney disease, overactive thyroid, diabetes or asthma.

  • Always check the label before giving an antihistamine to children as not all antihistamines are suitable for children. 

Any medication must be taken only after the prescription given by the doctor.

2. Can You Develop an Allergy for an Antihistamine Itself?

Yes, one can develop an allergy to any kind of medication, including antihistamines as each person’s body reacts in a different way to each kind of medication.  You must always consult a doctor before starting with any medication and should always adhere to the dose prescribed by the doctor. If you find that a drug does not work well for you, then instead of increasing its dose on your own, you must consult the doctor to try another drug rather than changing the dose without prescription. Consuming antihistamines in an overdose can cause serious side effects like tightness, respiratory depression, a burning sensation in the nose, or even death in worst cases.