The following text clarifies the natural function of histamine in the body and the therapeutic use and mode of action of antihistamines in allergies and other disorders, as well as what their side effects are.
What are antihistamines?
Antihistamines are the most widely used drugs in the treatment of allergies. It is a set of drugs whose common feature is the inhibition of the effects of histamine.
Histamine is present in all body tissues and acts in various physiological processes from allergic reactions to the acid secretion of the stomach by the parietal cells of the stomach, and at the central nervous system level it regulates the appearance of sensations such as hunger and sleep cycles. -vigil.
How do they work?
Histamine has important physiological functions and acts through four types of receptors: H1, H2, H3, H4. Once histamine is released it can exert local or scattered effects on smooth muscles and glands.
It contracts many smooth muscles, such as the bronchial and intestinal muscles, but relaxes others, including the small blood vessels. It is responsible for the appearance of symptoms such as itchy nose and eyes, runny nose, redness of the skin, appearance of skin lesions such as hives or a rash.
The antihistamines inhibit H1 receptors and the name of antihistamines is reserved for this group of drugs used in the treatment of symptoms of allergic diseases.
In turn, there are also H2 receptor inhibitors which regulate the acid secretion of the stomach and are used in the treatment of gastritis, gastroesophageal reflux and ulcer.
They are used in the management and prevention of "motion sickness" (motion sickness) and vertigo, also in the treatment of insomnia, migraine and as appetite stimulants.
They work by binding to histamine H1 receptors, preventing their activation. A crucial aspect of the pharmacology of these agents is that they are not antagonists as such, but inverse agonists, in other words, they are agents that preferentially bind to the inactive state of histamine receptors, stabilizing it for hours. Classical 1st generation and non-sedating or 2nd generation classics are classified into two groups.
Side effects
The most common side effects of taking antihistamines include headache, drowsiness, dizziness, and dry mouth. The different side effects on the body are mentioned below.
About the Nervous System
Antihistamines are generally very well tolerated drugs, especially second generation drugs.
The most important adverse reactions due to their frequency and magnitude are those related to the Central Nervous System. They can cause depression of the Central Nervous System, which can range from sedation to induction of coma which is rare and uncommon.
It produces deterioration of the ability to perform simple or complex tasks, especially those of the first generation due to the fact that they more easily penetrate the nervous system even at a minimal dose. These symptoms can even persist until the morning after the nocturnal administration of the drug, when the sedation is no longer perceptible, causing a “hangover” effect.
Other reported manifestations include tinnitus or a ringing sensation in the ears, double or blurred vision, dizziness, fatigue, difficulty in coordination, euphoria, nervousness, tremors, and pupillary dilation.
Cardiac Toxicity
All antihistamines have the ability to alter cardiac function although very infrequently, being able to trigger tachycardia, hypotension, hypertension and cardiac arrhythmia. In the last decade of the 20th century, serious cardiac arrhythmias appeared in some patients treated with astemizole and terfenadine (first-generation antihistamines).
These could occur in situations of overdose or simultaneous administration with other drugs of liver metabolism, this problem led to the withdrawal from the market of the aforementioned active principles. And it has not been subsequently presented with other antihistamines.
Other effects
Manifestations associated with the possible anticholinergic action in some of these agents is quite frequent, in the form of dry mucous membranes, various urinary disorders and various digestive disorders such as nausea, abdominal pain, vomiting, constipation, diarrhea.
However, taking the drugs with food often reduces these effects. In very rare cases, a decrease in the white blood cell count, agranulocytosis, hemolytic anemia, photosensitivity, difficulty in focusing on objects, increased liver enzymes, and hepatitis have been reported.
The antihistamines stimulate appetite through its action on H1 receptors in the central nervous system, this effect is more common in first generation antihistamines and for many years they were used in many drugs that stimulate appetite.
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