Antihistamines

Antihistamines are medicines used to treat allergy symptoms. This group of drugs inhibits the effects produced by histamine.

Histamine is involved in many physiological processes in the body by binding to various receptors when it detects a foreign allergen; antihistamines inhibit the H1 receptors.

Antihistamines for allergy are used to treat symptoms caused by histamine action: rhinitis, allergic conjunctivitis, hives, itchy skin of various causes, irritative cough, insect bites, colds, nausea and dizziness, lack of appetite and minor sleep problems.

There are a multitude of antihistamines marketed to treat allergy symptoms, with very different compositions, mechanisms of action and side effects. The physician will determine which is most appropriate for each type of allergy on a case-by-case basis.

Clinically, antihistamines are classified into two groups:

• Classical or first-generation antihistamines: these are molecules that penetrate well into the CNS and are not very selective in their actions. They are used mainly for allergic rhinoconjunctivitis, itching, hives, cough, nausea, prevention of motion sickness and insomnia. They are rapidly metabolised, so the dosage is usually 3 or 4 times a day.
• Non-sedative or second-generation antihistamines: these are drugs derived from the previous ones, but with a more selective action and less distribution. They are used for allergic rhinoconjunctivitis and urticaria, being the first choice in these cases, but not for other purposes (dizziness, insomnia...). They are usually taken once a day.

The most commonly used first-generation antihistamines are:

• Hydroxyzine, used as an antihistamine and as a tranquilliser.
• Chlorpheniramine and its analogue, dexchlorpheniramine, are available orally, topically and by injection.
• Diphenhydramine, used as both a sleep inducer and an antihistamine, and available in many countries by mouth and injection; and its derivative, dimenhydrinate, used for motion sickness.
• Promethazine, also used to prevent and treat nausea and dizziness, and also available orally and intravenously.
• Azatadine and its derivative, cyproheptadine, the antihistamine most commonly used in years past as an appetite stimulant.
• Ketotifen, most commonly used in children, is available orally and in eye drops.

There are many other first-generation antihistamines, most of which are part of over-the-counter anti-catarrhal compounds (Bronchostop, Frenadol, Vincigrip, etc.). Doxepin, an antidepressant with a potent antihistamine and sedative action, is still used for some indications of classical antihistamines.

The most commonly used second-generation antihistamines are bilastine, cetirizine, desloratadine, ebastine, fexofenadine, levocetirizine, loratadine, mequitazine, mizolastine and rupatadine, all of which are taken orally, either in capsule or syrup form. In addition, there are several marketed only topically, as eye drops and/or nasal sprays, such as alcaftadine, azelastine, bepotastine, emedastine, levocabastine or olopatadine.

At present, taking antihistamines in pregnancy is not considered 100% safe, regardless of the active substance chosen. Therefore, before using these drugs, you should try to reduce the symptoms with other means, such as avoiding the allergen causing the symptoms, seawater irrigators, saline solution... and if this is still necessary, you should consult your doctor to see what recommendation he/she makes in this regard. First generation antihistamines are usually used as antiemetics to counteract nausea.

Alcohol and antihistamines are not recommended, as they can cause dizziness and drowsiness, increasing the risk of overdose and accidents.

Antihistamines act on the central nervous system, which can cause various side effects such as drowsiness, tiredness, blurred vision, constipation, etc.

First-generation antihistamines tend to produce more adverse effects; even some, such as Dormidine, whose active ingredient is an antihistamine, are commonly used to produce drowsiness in cases of occasional insomnia.

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