Chlorphenamine is a first generation antihistamine used to treat different allergic manifestations such as seasonal rhinitis, conjunctivitis, urticaria and other symptoms. In addition, along with epinephrine, it is used in anaphylactic reactions.
We will see the following points about chlorphenamine:
- Mechanism of action.
- Pharmacological actions.
- Adverse reactions.
Mechanism of action
Chlorphenamine is an antihistamine drug that reversibly and competitively inhibits the binding of histamine to its H1 receptors.
There are 4 different receptors for histamine. H1 is found in vascular, intestinal and bronchial smooth muscle . We also find this type of receptors in the cardiovascular system and in the brain.
When histamine is coupled to this receptor, phospholipase C is stimulated and as a consequence, intracellular calcium increases. It also stimulates the enzyme nitric oxide reductase and releases arachidonic acid.
All these actions are aimed at producing vasodilation, hypotension, increased permeability and production of edema . In addition, it is typical of the activation of this receptor the triple Lewis response , characterized by redness, erythema and papule.
These manifestations are typical of allergic processes dependent on the activation of H1 receptors, therefore, when chlorphenamine is administered we suppress them by their antagonistic action.
Chlorphenamine and other first generation antihistamines produce, to a greater or lesser degree, sedation and drowsiness. This fact means a limiting factor for its continued use.
Among its pharmacological actions, those related to peripheral H1 antagonism predominate . It well antagonizes the increase in capillary permeability, pruritus, bronchoconstriction and intestinal contraction when strictly produced by histamine.
Only partially antagonizes hypotension and edema secondary to vasodilation , since this also exists a H2 component.
In the central nervous system (CNS) the sedative and hypnotic action predominates, which also varies according to the person. It also produces antiemetic, anticinetic and anti-fungal effects. To toxic doses usually produces an intense central stimulation, developing convulsions, although these effects we will see them later.
In children a picture of excitement and agitation may occur. It can also trigger some anticholinergic action causing dry mouth and mucous membranes.
Chlorphenamine is usually administered orally since it is absorbed fairly well but its bioavailability is less than 50%. In addition to this route, it can also be administered cutaneously, intramuscularly or intravenously.
The presence of food in the stomach causes the absorption of chlorphenamine to be delayed, but does not affect its bioavailability. This parameter is so low because it suffers a strong first-pass effect.
The beginning of the antiallergic action of chlorpheniramine is observed at 30-60 minutes and the maximum at 6 hours. The maximum plasma concentrations of this drug are observed at 2 hours after administration and the duration of the effects varies between 4 and 8 hours.
Chlorphenamine suffers from a strong hepatic metabolism , first in the same gastric mucosa and then in its first passage through the liver.
The elimination half-life varies according to the age of the patients. In healthy adults it is 20 to 24 hours, while in children it decreases by half. In patients suffering from renal insufficiency, the elimination half-life depends on the degree of the insufficiency and can reach 3oo hours or more.
This antihistamine binds to the proteins of the plasma in a 72% , reason why it is necessary to have precaution if it is taking some other drug that also a plasmatic proteins in a high percentage, since it can displace to the clorfenamina and thus pontenciando effects coming to produce toxicity.
It is a drug capable of crossing the placental barrier, forcing it to use it with caution in case of pregnancy or suspicion of it.
The adverse effects of chlorphenamine are abundant and relatively frequent. They depend on the dose and the patient . The most frequent reactions are those that are caused by CNS depression, such as:
Due to these adverse effects, patients should be warned that this drug can significantly affect their mental capacity when driving a vehicle or operating a machine.
These manifestations may disappear after a few days, but if they persist it is necessary to consider a reduction of the dose or the change to another antihistamine.
Chlorphenamine has, as mentioned above, a certain anticholinergic activity that can be translated into:
- Dry mouth, nose and throat
- Blurry vision
- Urinary retention
It can also produce adverse reactions in the digestive system such as nausea , vomiting, loss of appetite or diarrhea . In addition to producing tachycardia, hypotension or hypertension , extrasystoles and arrhythmias.