Definition
Cetirizine is a medicinal substance known as a second-generation antihistamine. Drugs containing cetirizine are frequently used in the treatment of allergies. Cetirizine is offered in different dosage forms, whereby the drugs are freely available in pharmacies, i.e. they are not subject to prescription. The prices vary depending on package size and manufacturer, whereby the available generic drugs are sometimes offered at many times lower prices than other available preparations containing the active ingredient cetirizine.
Mode of operation
Pharmacologically, cetirizine can also be called an H1-receptor antagonist. This means that cetirizine blocks a specific receptor, which is normally activated by histamine. When histamine binds to the H1 receptor, this has a number of different effects in the body.
Especially in allergies, this receptor plays a crucial role. Overall, this receptor is widely distributed in different structures in the body. Thus, the receptor could be localized in smooth muscles, nerve cells, as well as in cells of the immune system.
The H1 receptor is thus found in the smooth muscles of vessels and, when activated by histamine, ensures increased vascular permeability (increased permeability of fluid) and dilation of the vessels. This explains the connection that in allergies such as hay fever, where there is a lot of histamine in the blood, the nose usually runs. However, when the H1 receptor is blocked by cetirizine, the histamine can no longer bind to the receptors and the allergic symptoms ideally disappear almost completely.
Histamine also mediates the itching that often occurs in certain skin diseases or allergies. Thus, a blockade of the receptors by cetirizine can also significantly improve this symptom. Another important effect of cetirizine is its effect on histamine receptors in the smooth muscles of the bronchi.
Without cetirizine, histamine can bind to these receptors, causing the muscles in the bronchi to contract and breathlessness to develop. For this reason, especially in allergies where histamine is present in large quantities in the blood, shortness of breath can develop, which can be prevented with cetirizine. Cetirizine is a second generation antihistamine.
In contrast to the first generation of antihistamines, cetirizine is hardly present in the brain after ingestion and thus causes significantly fewer side effects in the form of fatigue than was the case with the first generation. The difference between the two generations of antihistamines is the inability of the second generation to cross the so-called blood–brain barrier. This is of great importance because the brain also contains histamine receptors that can be blocked by cetirizine. When the H1-histamine receptors in the brain are blocked, the “wake-up system” in the brain is blocked and thus fatigue is triggered. However, since cetirizine does not cross the blood-brain barrier, this fatigue does not, or only rarely, occur when taking cetirizine.