Promethazine is a drug in the phenothiazine family, used in the treatment of mental and emotional disorders and to reduce nausea. Its activity is based on reducing the levels of histamine in the body. Promethazine has no neuroleptic effects, although it is very useful in the symptomatic treatment of allergies, reducing the discomfort of rhinitis and allergic conjunctivitis.
Histamine and allergy
Histamine is a brain neurotransmitter (chemicals released during neuronal communication) and an endogenous amine. It is stored in granules that can be found inside some cells, such as mast cells and basophils, very related to allergic reactions; and on platelets.
Histamine participates mainly in inflammatory processes, but also has receptors in the stomach and brain mucosa. This explains the effect of antihistamines such as promethazine in the central nervous system (they are hypnotics) and in the prevention of nausea and vomiting.
Promethazine is going to be used mainly as a treatment for allergies . Allergies occur because our defenses act against exogenous substances (allergens) in an exaggerated way , such as pollen or certain components of the fruit. Therefore, they are also called hypersensitivity reactions.
When the allergen enters the body, the immune system recognizes it as foreign and attacks it giving rise to cells specialized in the elimination of said substance. Consequently, when the allergen is back in the body, these cells are activated, developing the allergy picture.
Among the mechanisms involved, in an allergic reaction is very important mass release of histamine from cells such as macrophages and mast cells. Therefore, reducing histamine levels is a way to reduce the discomfort caused by this pathology.
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Mechanism of action of promethazine
Promethazine is an antihistamine, that is, it decreases the action of histamine . It does this by binding to a histamine receptor, blocking it and preventing histamine from activating it and causing allergic symptoms. It does not prevent the allergic reaction, it only reduces its symptoms, that’s why it is said to be a symptomatic treatment.
It is administered orally and rectally. To treat allergic reactions, usually one to four oral doses a day are prescribed. When used in the treatment of nausea due to travel, it should be taken approximately one hour before leaving. The main problem with antihistamines is that due to their effect on the nervous system, they are hypnotic and produce a lot of drowsiness.
Indications of promethazine
Allergic rhinitis is an inflammation of the nasal mucosa caused by exposure to an allergen, that is, a substance harmless to the body. The release of histamine in this context will intervene in all the symptoms that characterize this pathology:
- Itching or nasal itching. It is the most characteristic symptom of allergic rhinitis. It is produced by irritation of small nerves in the area.
- Sneezing Produced by a reflex mechanism triggered by the substances released.
- Mucosal discharge or rhinorrhea due to a glandular stimulation mediated by histamine.
- Obstruction of the airway. Because the vessels in the area dilate as a result of the release of mediators of inflammation.
Therefore, antihistamines such as promethazine are the first line of symptomatic treatment of allergic rhinitis.
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Conjunctivitis is an inflammation of the mucous layer that lines the eyeball, called the conjunctiva. In this case the release of histamine and other mediators of allergic reactions produce ocular involvement, characterized by:
- Mucous tearing , with a whitish, liquid and very abundant secretion.
- Redness of the conjunctiva , that’s why allergy sufferers have red eyes.
- The eyelids are swollen and sore or, more technically speaking, eyelid edema or chemosis .
- There is usually associated rhinitis .
In the same way as in rhinitis, promethazine and other antihistamines are the first line of treatment in allergic conjunctivitis , reducing the discomfort caused by the symptoms.
As we have said, histamine receptors are not only in inflammatory cells. They also exist in other tissues such as the gastric mucosa and the central nervous system. This means that promethazine can be used in other areas such as the management of postoperative nausea and vomiting, travel sickness, or as a hypnotic.