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General Practice 2023

Anti-histamínicos: what they are and main types

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Anti-histamínicos: what they are and main types
Anti-histamínicos: what they are and main types

Antihistamines are remedies indicated for the treatment of allergic reactions, such as hives, runny nose, allergic rhinitis or conjunctivitis, for example, as they act by reducing the symptoms of itching, swelling, redness or runny nose. These remedies work by preventing the action of histamine, a substance that causes allergies, and are therefore also called antiallergic.

The type of antihistamine that can be used depends on the condition to be treated, and the doctor may indicate the use of first, second or third generation antihistamines, which have differences in the form of action and side effects such as drowsiness for example.

Therefore, it is important to consult the doctor so that the diagnosis of the condition to be treated is carried out and, thus, the best antihistamine, the dose of the medication and the duration of treatment are advised.In addition, these remedies should not be used by children, pregnant or breastfeeding women without medical advice.

The main types of antihistamines include:

1. Classic or first-generation antihistamines

Classic or first-generation antihistamines were the first to be introduced on the market and act by blocking the release of histamine in the brain and spinal cord. Therefore, they cause more side effects such as severe drowsiness, sedation, fatigue, changes in cognitive functions and memory. In addition, they are also more difficult to eliminate from the body and, for these reasons, should be avoided.

The main first-generation antihistamines are:

  • Promethazine, tablet or skin cream (Phenergan or Profergan);
  • Dexchlorpheniramine, tablet, syrup, drops or dermatological cream (Polaramine or Histamin);
  • Hydroxyzine, tablet, syrup or oral solution (Hixizine or Pruri-gran);
  • Clemastine, tablet or dermatological cream (Emistin);
  • Diphenhydramine, injectable ampoule (Diphenhydrin).

These antihistamines are generally indicated for the treatment of allergic reactions, allergic dermatitis, urticaria, allergic rhinitis, vertigo, nausea caused by travel, or prevention of postoperative vomiting, for example, and should be used with medical advice.

In addition, some first-generation antihistamines, such as diphenhydramine, can be used in hospitals to treat severe allergic or anaphylactic reactions, applied directly into a vein or muscle by a nurse under medical supervision.

2. Non-classical or second-generation antihistamines

Non-classical or second-generation antihistamines are drugs that act by preventing the action of histamine in peripheral tissues, such as blood vessels, gastrointestinal tract and respiratory system.These antihistamines penetrate the central nervous system to a lesser extent and are eliminated more quickly, therefore presenting fewer side effects.

The main second-generation antihistamines are:

  • Loratadine, tablet or syrup (Claritin, Histadin or Loratamed);
  • Cetirizine, tablet, capsule or oral solution (Zyrtec or Reactine);
  • Bilastina, tablets or oral solution (Alektos);
  • Ebastina, tablet or syrup (Ebastel);
  • Epinastine, tablets or eye drops (Talerc or Relestat).

These antihistamines are usually indicated for allergic rhinitis, hives, allergic rhinoconjunctivitis, helping to relieve symptoms such as itchy skin, eyes or nose, a runny or stuffy nose, sneezing, red eyes or coughing.

Before starting treatment with second-generation antihistamines, consult a doctor, so that the most appropriate treatment can be recommended according to the symptoms presented. Learn how to recognize allergy symptoms.

3. Third-generation antihistamines

Third-generation antihistamines are medicines produced by modifying molecules of second-generation antihistamines, presenting the same action and efficacy, but with lower doses and fewer side effects.

The main third-generation antihistamines are:

  • Levocetirizine, tablets or drops (Zyxem, Zina or Rizi);
  • Desloratadine, tablet or syrup (Desalex, Leg or Esalerg);
  • Fexofenadine, tablet or syrup (Allegra, Fexx or Allexofedrin).

These remedies are indicated for the treatment of seasonal allergic rhinitis, chronic urticaria, skin allergies, runny nose, allergic conjunctivitis or hay fever, for example, and should be used for the duration of treatment advised by the doctor.

Which antihistamines can be used in pregnancy

During the entire pregnancy, the use of medications, including antihistamines, should be avoided as much as possible, especially in the first trimester of pregnancy, as this is the phase in which the cells of the embryo rapidly multiply and the main cells form organs, and the use of drugs or other substances can affect the baby's development.

However, if necessary, the pregnant woman can take antihistamines, but only if indicated by the obstetrician. The antihistamines that are considered safer in pregnancy, and that are in category B, are chlorpheniramine, loratadine and diphenhydramine.

When not to use

Antihistamines can be used by anyone, however, there are some cases that need medical advice such as:

  • Pregnancy and breastfeeding;
  • Children;
  • Glaucoma;
  • High blood pressure;
  • Kidney or liver diseases;
  • Benign prostatic hypertrophy.

In addition, some of these medications may interact with some anticoagulants and central nervous system depressant medications, such as anxiolytics or anti-depressants, and it is advisable to consult a doctor before using.

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