Antihistamines: What Are They For?

In addition to the treatment of allergies, some antihistamines are also indicated to prevent vomiting and dizziness or to induce sleep in those suffering from insomnia.

Antihistamines are used to reduce or eliminate the effects of allergies. This effect is possible because they block the receptors to which histamine joins in allergic reactions.

Histamine, as we will see later, is a substance released by the body when an allergic reaction is triggered.

Following the discovery in the 1930s and 1940s of the first antihistamines (pyrilamine and diphenhydramine), hundreds of molecules with antihistamine properties have been developed. Furthermore, the use of these drugs has grown in recent years. 

Likewise, the introduction of second and third generation antihistamines  represented a great advance in medicine. These medicines have similar efficacy to first generation antihistamines, but have much more moderate adverse effects.

On the other hand, in addition to treating allergies, some antihistamines can also be used to prevent vomiting and dizziness or to induce sleep in people with insomnia.

Histamine and its receptors

Histamine is a chemical that participates and intervenes in many processes that occur in cells. Among these, its role in allergic reactions, inflammatory states, in the secretion of gastric juices and in the transmission of nerve impulses stands out . 

In order to intervene in all these physiological processes, it joins a series of receptors that are distributed throughout the body.

When histamine joins one of these structures, the organism activates mechanisms and secretes substances that will activate and join other structures in order to trigger a physiological action.

Depending on the receptor to which it joins, different reactions will be triggered. As for the receptors to which histamine joins, we can mention the following:

• Histamine H1 receptor:  These structures are found in the bronchi, smooth digestive tract and brain. When these receptors are activated, a construction is produced in both the smooth muscle of the bronchi and blood vessels.

• H2 histamine receptor : H2  receptors are distributed in the lining of the stomach, uterus and brain. When activated, the permeability of blood vessels also increases and the secretion of gastric juices is stimulated .

• Histamine H3 receptor:  These are located in the brain and bronchi. They are responsible for cerebral vasodilation and may be involved in a retro-feeding or negative feedback system, for which histamine inhibits its synthesis and release from nerve endings.

Classic or first generation H1 antihistamines

These antihistamines easily cross the brain’s blood brain barrier, a kind of membrane that protects the brain. They thus join the receptors of this organ, triggering intense sedative effects.

In addition, they  also exhibit anti-blood and anticinetoside properties. These actions are due to the sedative and anticholinergic effects of these medicines. Some drugs that are included in this therapeutic group are as follows:

• Ethanolamine: detaches the drug diphenhydramine. Its derivative, dimenhhydrinate, has anticinetoside properties and is used to prevent dizziness while traveling. There are studies on the effectiveness of this active ingredient for the treatment of vertigo and in the prophylaxis of vomiting after an operation.
• Ethylenediamines.
• Alkylamines.
• Piperazine.
• Phenothiazines.
• Piperidine .

Second generation H1 antihistamines

Second generation H1 antihistamines are more selective drugs for H1 receptors which are not found in the brain,  as they are unable to cross the blood brain barrier. 

The reason why it happens is that in its chemical structure the parts that allowed it to cross this barrier with ease have been eliminated.

Consequently, they  trigger fewer adverse effects; produce less sedation and fewer anticholinergic effects. Some examples of these medicines are:

• Loratadine : is an antiallergic drug that has almost no sedative and anticholinergic effect.

• Ebastine: They  do not have anticholinergic or sedative properties, but can cause weight gain. It is useful in the treatment of seasonal allergies.

• Desloratadine: it  is used, above all, in acute cases of allergy.

Widely used drugs

The main indication of antihistamines is the treatment of allergies. Yet, they can also be used to prevent vomiting or to induce sleep in patients suffering from insomnia.

Consult your doctor about the best antihistamine for your personal situation and always follow his advice.