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Antihistamines are medications that are often purchased at pharmacies with or without a prescription to treat different types of allergies. This drug is useful in reducing the occurrence or severity of allergic symptoms caused by exposure to the allergen. One of the frequent consequences of the first developed antihistamines was that they caused sleepiness. But how can treating allergy symptoms have such side effects?
Secondary effect: a molecule that activates many mechanisms
Antihistamines are designed to counteract the effect of histamine, which is released by our cells during an allergic reaction. Once histamine is secreted, it is capable of activating other cells – by binding to areas on their surface called receptors – and triggering a chain reaction. Histamine is partly responsible for the wide variety of allergic symptoms because there are four types of histamine receptors spread over many cell types .
These receptors (called H 1 -H 4 ) are found on different cells such as cells of the immune system (neutrophils, eosinophils or mast cells), muscle, endothelial or epithelial cells, and even neurons. However, not all cells have all four types of receptors, so depending on which cell receives the histamine molecule, one mechanism or another is activated .
As each cell has certain functions, the activation caused by histamine can cause dilation of blood vessels, constriction of the bronchi , recruitment of cells of the immune system, control over gastric juices or even sleep cycles .
In each context, histamine is responsible for regulating vital processes in the body or eliminating pathogens . In other words, histamine in the right amounts and at the right time is good for our bodies. Therefore, when developing an antihistamine, all its functions must be taken into account so as not to upset the delicate balance between functions.
What does an antihistamine do?
In the case of neurons, histamine is a molecule that causes an excitation, that is, it keeps us active in a state that we know as “wakefulness” and reduces sleep.
Antihistamines are “plugs” on these histamine receptors, preventing them from being activated . The progress in the development of this medicine is based on “covering” only one type of receptor, or doing it optimally.
Due to their ability to penetrate the brain, some antihistamines are capable of blocking the action of histamine on neurons. By blocking it, that ability to stay active is reduced, triggering that side effect that makes us feel tired or sleepy.
Antihistamines today are used to treat various types of allergies such as rhinitis, conjunctivitis , in some cases of non-anaphylactic food allergies and in a percentage of hives . However, they can also be used as a remedy against motion sickness (the motion sickness that some people experience when traveling by car or boat).
Over 100 years of antihistamines
The first antihistamine compounds were developed in the early 1930s at the famous Pasteur Institute , France. When they finally hit the market in the 1940s, they were primarily intended to treat allergy symptoms. However, it was believed that they were also effective against colds, so their use spread tremendously rapidly throughout the population.
In some studies, the use of first-generation antihistamines was shown to affect the school performance of children who took them to curb allergy symptoms. They have also been found to cause plane, boat, and car accidents , when compared to the use of non-sedating antihistamines.
The sedative effect has caused associations such as GA 2 LEN, the Global Allergy and Asthma European Network to advise against them as the first line of treatment for allergies . The good news is that second-generation, non-sedating antihistamines have been shown to be safe and effective.
However, first-generation antihistamines can still be purchased at the pharmacy without a prescription. Some people who have been using one type of drug for many years are reluctant to try another drug for fear that it might not work as well.
Over the decades and advances in the study of histamine and its receptors, the development of antihistamines has been refined to eliminate side effects. Today, there are second and even third generation antihistamines, which are much more specific and are better assimilated.
Over-the-counter antihistamines
Since antihistamines can be obtained at the pharmacy without a prescription, it is important to make proper use of them. The first thing is to try to avoid the first generation , which can have a sedative effect. If you are a person who suffers from allergies and used to take this version, you can consult your specialist to find out what alternatives exist .
Antihistamines are not effective against the symptoms of the common cold . Colds are infections caused by different types of viruses and although they are uncomfortable, our immune system is capable of ending the infection on its own.
There is no evidence that long-term use of antihistamines triggers tachyphylaxis , commonly known as drug tolerance, especially for the newer generation antihistamines. Therefore, it is not necessary to alternate medications or increase the dose to ensure its effectiveness.
Finally, and as we always advise, in case of doubt, it is best to consult a specialist or family doctor about the available options that best suit each disease.
References:
Church et al. 2010. Risk of first-generation H1-antihistamines: a GA2LEN position paper. allergies. doi: 10.1111/j.1398-9995.2009.02325.x
Parsons and Ganellin. 2006. Histamine and its receptors. Br J Pharmacol. doi: 10.1038/sj.bjp.0706440
Randall et al. 2018. Antihistamines and allergies. Aust Prescr. doi:10.18773/austprescr.2018.013
