• Wednesday April 8,2020


Antihistamines, histamine receptor antagonists or histamine receptor blockers, are medicines used to treat allergic reactions in order to neutralize the effect of the body's histamine. Antihistamines were discovered as early as 1937 and for the first time in 1942 were also used therapeutically.

What are antihistamines?

Antihistamines are used in allergic immune reactions of the body to lift the effect of histamine.

Antihistamines are used in allergic immune reactions of the body to lift the effect of histamine. Histamines bind to receptors to trigger an immune response in the body. Antihistamines block the docking sites of receptors, of which there are four different types: H1, H2, H3 and H4 receptors.

Histamine is an endogenous hormone and is found in inactive form, especially in the mast cells and in the leukocytes, which are part of the immune system. If the body is exposed to antigens - extraneous, allergy-causing substances - they become attached to the leukocytes or to the so-called immunoglobulin E, which is located on the surface of the leucocytes.

The leukocytes are thereby destroyed and the stored histamine released therein. To reduce the effects of histamine release and to prevent further secretion of histamine, antihistamines are prescribed and administered by the doctor.

Application, effect & use

Antihistamines are used in allergic reactions. Antihistamines not only block the receptors so that histamines can not bind to them again, it also works against the histamine that has already been released by the leukocytes. The receptors are divided into four groups: H1, H2, H3 and H4 receptors.

The H1 receptors cause the following reactions in the body: The blood vessels dilate, resulting in a fall in blood pressure. The vessel walls become more permeable. As a result, in addition to redness and edema (water retention) on. As the blood vessels dilate, the H1 receptors in the bronchi cause the opposite effect.

In particular, asthmatics are at risk because the bronchi can be life-threatening constrict. In addition, the H1 receptors stimulate nerve impulses, making the skin hypersensitive to touch and itching.

When the histamines bind to the H2 receptors, this causes reactions in the cardiopulmonary circulation system. The heart rate increases and the pulmonary vessels expand. Furthermore, they have an inflammatory effect on the gastric mucosa and stimulate gastric acid production, which can lead to gastritis and heartburn.

When histamine binds to H3 receptors, self-regulating processes occur. Histamine secretion is inhibited. Research on H4 receptors is still in its infancy, but it is thought to have an effect on allergic asthma.

Antihistamines reverse the action of the hormone histamine. Because of this, there are two types of antihistamines: H1 and H2 antihistamines. H1 antihistamines are mainly used for hay fever, urticaria (hives), as well as in other allergic reactions (watery, itchy eyes, runny nose, shortness of breath, etc.).

H1 antihistamines have a spasmolytic (anticonvulsant) as well as vascular sealing effect. The already dilated blood vessels narrow, the permeability of the vessel walls is reduced, so that edema, skin redness and itching regress. H2-antihistamines block the H2-receptors so that no inflammatory reactions in the stomach can be caused. H2-antihistamines inhibit the production of stomach acid.

Depending on which active ingredient is used, its effect usually sets in between 30 and 60 minutes. After about three hours, the max. Efficacy is achieved and usually lasts for one day, with the effect steadily decreasing over the course of the hour.

In addition to the treatment of allergic reactions, antihistamines are also used to treat gastric ulcers, ADHD, sleep disorders and Alzheimer's.

Herbal, natural & pharmaceutical antihistamines

Antihistamines are currently only marketed as H1 and H2 antihistamines and are divided into so-called three generations: 1st generation, 2nd generation and 3rd generation antihistamines.

The 1st generation of antihistamines include the following active ingredients: Bamipin, Clemastine and Dimetinden, Promethazine, Diphenhydramine, Ketotifen and Dimenhydriant. These medicines have many side effects. Because of this, they are no longer used in oral form (tablets, etc.). The application mainly takes place externally using ointments, drops, gels and creams.

With the development of antihistamines of the 2nd generation, the above-mentioned side effects were reduced or no longer occur. Second generation drug classes include azelastine, cetirizine, loratadine, levocabastine, fexofenadine, and mizolastine.

The dosage forms are tablets, capsules, sustained-release tablets, ointments, nasal sprays, eye drops and, in the case of acute and severe allergic reactions, solutions for injection or infusion. Some of the antihistamines are available over-the-counter in the pharmacies (mainly the second generation), but there are also prescription (1st generation) products that must be prescribed by the doctor.

In addition to the chemical-pharmacological products, there are also natural antihistamines, which in combination can reduce the allergic reaction of the body. Ascorbic acid, ascorbate and ascorbyl palmitate (vitamin C) help to break down the histamine faster. Pantothenic acid (vitamin B5) is an important building block in the production of cortisol in the adrenal glands. Cortisol has anti-inflammatory properties. Calcium and zinc can block the docking sites of the receptors so that the histamine can not bind. Manganese can block the release of histamine and accelerate the breakdown of histamine.

Flavonoids are antioxidants that can have an anti-inflammatory effect. The flavonoids hesperidin, rutin and quercetin may have a stabilizing effect on the mast cells, so that they can not be destroyed by the antigens and the histamine can not be released.

Risks & Side Effects

1st generation antihistamines have many side effects. H1 antihistamines have good CNS mobility, that is, they can cross the blood-brain barrier so that they act directly in the brain and spinal cord. As a result, side effects such as tiredness, low blood pressure, rapid heartbeat, headache, nausea, vomiting and impairment of liver and kidney function may occur.

Since antihistamines of this group have a sedating (tired) effect, the ability to drive and use machines is severely limited. If cardiac arrhythmia, glaucoma, epilepsy, asthma and liver and kidney dysfunction are present, 1st generation H1 antihistamines must not be taken as they may be beneficial for these conditions. Antihistamines should not be taken during pregnancy and lactation.

2nd generation antihistamines can no longer penetrate the blood-brain barrier, significantly reducing the side effects. However, the above-mentioned side effects can also occur here, but their occurrence is much less common.

Side effects can also occur with natural antihistamines. An overdose of vitamins and minerals can cause cardiovascular disease (including heart attack) as well as kidney and liver dysfunction.

Interactions with other drugs

First-generation antihistamines, in conjunction with tricyclic antidepressants, can lead to glaucoma (glaucoma). Drugs from the drug groups azelastine and cetirizine should not be combined with each other as the cardiovascular diseases may be the result of the interactions.

Antihistamines should not be taken with analgesics (analgesics), sleeping pills and anesthetics. H1 and H2 antihistamines should not be taken together with beta blockers and ACE inhibitors (antihypertensive medicines) or with blood coagulants (warfarin).

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