Antihistamines – Effect, Application & Risks


Antihistamines , histamine receptor antagonists or histamine receptor blockers , are drugs used to treat allergic reactions to neutralize the effects of the body’s histamine. Antihistamines were discovered in 1937 and used therapeutically for the first time in 1942.

What are antihistamines?

Antihistamines are used when the body is having an allergic immune response to counteract the effects of histamine. Histamine binds to receptors to trigger an immune response in the body. Antihistamines block the docking sites of the 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 primarily in mast cells and leukocytes, which are part of the immune system . If the body is exposed to antigens – foreign, allergy-causing substances – these attach themselves to the leukocytes or to the so-called immunoglobulin E, which is located on the surface of the leukocytes.

This destroys the leukocytes and releases the histamine stored in them. In order to reduce the consequences of histamine release and to prevent further release of histamine, antihistamines are prescribed and administered by the doctor.

Application, effect & use

Antihistamines are used for allergic reactions. Antihistamines not only block the receptors so that histamines cannot bind to them again, they also work against the histamine that has already been released from 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 drop in blood pressure. The vessel walls become more permeable. As a result , edema (water retention) occurs in addition to reddening of the skin. As the blood vessels dilate, the H1 receptors in the bronchi produce the opposite effect.

Asthmatics in particular are at risk because the bronchial tubes can become life-threatening. Furthermore, the H1 receptors stimulate the transmission of stimuli in the nerves, so that the skin reacts overly sensitively to touch and itching occurs.

When the histamines bind to the H2 receptors, this triggers reactions in the cardiopulmonary circulatory system. The heart rate increases and the pulmonary vessels dilate. They also have an inflammatory effect on the gastric mucosa and stimulate gastric acid production, which can lead to gastritis and heartburn .

Self-regulating processes occur when histamine binds to H3 receptors. Histamine release is inhibited. Research into H4 receptors is still in its infancy, but it is thought that it may have an effect on allergic asthma .

Antihistamines counteract the effects 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 (nettle rash), as well as for other allergic reactions ( watery, itchy eyes , runny nose , shortness of breath , etc.).

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

Depending on which active ingredient is used, its effect usually sets in between 30 and 60 minutes. The maximum effectiveness is reached after about three hours and usually lasts for a day, with the effect steadily decreasing over the course of the hours.

In addition to treating allergic reactions, antihistamines are also used to treat stomach ulcers , ADHD , sleep disorders , and Alzheimer ‘s.

Herbal, natural & pharmaceutical antihistamines

To date, antihistamines have only been on the market as H1 and H2 antihistamines and are divided into three generations: 1st generation, 2nd generation and 3rd generation antihistamines.

First-generation antihistamines include the following groups of active ingredients: bamipin, clemastine and dimetinden, promethazine, diphenhydramine, ketotifen and dimenhydriant. These drugs have many side effects. Because of this, they are no longer used in oral form (tablets, etc.). The application is mainly external with the help of ointments, drops, gels and creams.

With the development of second-generation antihistamines, the side effects mentioned above have been reduced or no longer occur. Drug groups of the 2nd generation 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, injection or infusion solutions. Some of the antihistamines are available in pharmacies without a prescription (mainly 2nd generation), but there are also prescription preparations (1st generation) that have to be prescribed by a doctor.

In addition to the chemical-pharmacological products, there are also natural antihistamines, which in combination can reduce the body’s allergic reaction. Ascorbic acid , ascorbate and ascorbyl palmitate ( vitamin C ) ensure that histamine is broken down more quickly. 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 cannot take hold. Manganese can block the release of histamine and accelerate the breakdown of histamine.

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

Risks & side effects

First-generation antihistamines have many side effects. H1 antihistamines have good CNS penetration, which means they can cross the blood-brain barrier, allowing them to act directly in the brain and spinal cord. As a result, side effects such as tiredness , drop in blood pressure , tachycardia , headache , nausea , vomiting and impairment of liver and kidney function can occur.Since antihistamines in this group have a sedating (making you tired) effect, your ability to drive and use machines is severely restricted. If cardiac arrhythmia , glaucoma (glaucoma), epilepsy , asthma and liver and kidney dysfunction are present, first-generation H1 antihistamines must not be taken, as they promote these diseases. Antihistamines should not be taken during pregnancy and breastfeeding .

Second-generation antihistamines can no longer penetrate the blood-brain barrier, so the side effects are significantly reduced. However, the side effects mentioned above can also occur here, but their occurrence is much rarer.

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

Interactions with other drugs

First-generation antihistamines combined with tricyclic antidepressants can lead to glaucoma (glaucoma). Preparations from the active ingredient groups azelastine and cetirizine must not be combined with one another, as the interactions can result in cardiovascular diseases.

Antihistamines must not be taken together with analgesics ( painkillers ), sleeping pills and anesthetics. H1 and H2 antihistamines must not be taken together with beta blockers and ACE inhibitors (medicines against high blood pressure) or with blood clotting drugs ( warfarin ).

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.