What the doctor needs to know about modern antihistamines

Antihistamines – a group of drugs that are widely used in modern medicine. A doctor of any specialty should clearly navigate the features of their use. Knowledge of the principles of action of different groups of antihistamines enables the doctor to provide a differentiated approach to the elimination of allergic manifestations in each individual case of the disease. The first antihistamine substance, thyxoxydiethylamine, was synthesized in 1936 by pharmacologist Daniele Bove.

This drug was not suitable for practical use due to its toxicity, but served as the basis for subsequent experiments and discoveries of most modern groups of antiallergic drugs. About pharmacodynamic properties, classification of antihistamines, indications for use from the standpoint of evidence-based medicine – read on estet-portal.com later in the article. 

What are the pharmacodynamic characteristics of antihistamines

The main pathogenetic element of allergic diseases is the release of biogenic amine from mast cell granules – histamine. In the human body, the main depot of histamine is the granules of mast cells, the immunological effect on which leads to the release of histamine from them, and its combination with specific rhodopsin-like membrane proteins – N

1_,N2 _{– receptors.} 

Pharmacodynamic properties of antihistamines consist in blocking the effect of histamine at one of the stages:

Histamine biosynthesis;
Histamine liberation from mast cell granules;
Transmitter connections with H
• 1_,H2_-receptors.

For more information about the pathogenesis of allergic reactions, see the article:

How acute allergic reactions develop in the human body

What is the current classification of antihistamines 

Today, there are a large number of antihistamines with different pharmacodynamic characteristics, which creates some difficulties for the doctor in choosing the active substance in the prescription.

The following classification of antihistamines is currently used:

First generation drugs (sedative effect is pronounced).

Second generation drugs (sedation is weak or absent):

1) Direct acting: H1_{, H}2_{-histamine blockers (H}1_{-receptor blockers are predominantly used)}

2) Indirect: cromoglycic acid, ketotifen, histaglobulin.

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In addition to the severity of the sedative effect, there are other differences between the groups of antihistamines: drugs belonging to the group of antihistamines of the second generation are highly selective for H1-histamine receptors, due to which they do not cause tachyphylaxis, have a longer period of therapeutic effect, do not pass through the blood-brain barrier and do not bind to other receptors. These advantages of using a new generation of antihistamines make them the drugs of choice in most cases - except in situations where parenteral administration of antihistamines is required. In such cases, representatives of the 1st generation are used. 

What are the indications and contraindications for prescribing antihistamines of the 1st generation

Despite the fact that antihistamines have a large number of positive effects, their use must be justified in each individual case. Unreasoned use of antiallergic drugs as part of "complex therapy" diseases – unacceptable. Indications for the use of antihistamines I are the following conditions:

Serum sickness;
Quincke's edema;
Anaphylactic shock;
Acute generalized urticaria;
Severe itching in contact or atopic dermatitis;
Pathological reactions to drug administration;
Insensitivity to 1st generation drugs.

Contraindications to the use of 1st generation antihistamines include the following conditions:

1. Bronchial asthma;
Glaucoma;
Prostate adenoma;
Acute urinary retention.

These conditions should be taken into account by the doctor before prescribing antihistamines.

What groups of drugs should not be combined with antihistamines

Each doctor who includes an antihistamine drug in the therapy regimen should collect a detailed life history from the patient and find out what concomitant pathological conditions he has and what groups of medications he is currently taking. It is necessary to be aware of the existence of unacceptable combinations of antihistamines with other drugs.

These include:

• Combinations of all antihistamines of the first generation with hypnotics, sedatives, anticonvulsants, MAO inhibitors;

• Combinations of 1st generation antihistamines (chloropyramine) with tricyclic antidepressants;

• Combinations of second-generation antihistamines of indirect action with acetylsalicylic acid, erythromycin;

• Combinations of 1st generation (terfenadine) and 2nd generation (loratadine) antihistamines with ketoconazole, itraconazole, erythromycin;

• Combinations of 1st generation antihistamines (diphenhydramine) with vitamin C, gentamicin.

Why does a doctor need to navigate the modern groups of antihistamines 

It is necessary to remember the rule: if an antihistamine is not indicated – it is contraindicated. 

This principle will help the doctor avoid polypharmacy, which is unacceptable in prescriptions. The ability to draw up effective treatment regimens for diseases, including the processes of allergic genesis, in accordance with the postulates of evidence-based medicine is an essential feature of a professional doctor. Thank you for staying with estet-portal.com. Read other interesting articles on the site in the "Dermatology" section.

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