What is an Antihistamine? Types, Side Effects & More (2022)

‍An antihistamine is a kind of medication that’s used to treat allergies. Antihistamines work by blocking histamine receptors in the body. This means that when you take an antihistamine, there is less histamine response and your allergy symptoms reduce. There are lots of different types of antihistamines, all with slightly different properties and uses.

We have lots of articles about what antihistamines do and how they work, as well as their side effects. If you would like to know more about any of these topics, just search for them on our site. In this article, however, we will focus on what exactly an antihistamine is from a chemical standpoint and what the main kinds of antihistamines are.

Types of Antihistamines

First, before we go into the different types of antihistamines, it’s essential to understand how they work. All antihistamines, whether in the form of a tablet, a liquid, or an injection, work by blocking histamine receptors in your body. 

Oral Antihistamines: Oral antihistamines are the most commonly used antihistamines. They come in the form of tablets that you can take orally. They’re broken down by your body quickly, so they have a short half-life (the time it takes to metabolize half the drug in your system). This means they only stay in your system for a few hours. 

Topical Antihistamines: Topical antihistamines are applied directly to the skin and are often combined with a substance that works as a local anesthetic. These are usually in the form of creams, sprays, shampoos, or ointments

Injectable Antihistamines: Injectable antihistamines are administered via an injection. They usually come in a liquid form, although some are in lyophilized powder form.

How do Antihistamines Help with Allergies?

Most allergies are caused by an excessive immune response to harmless substances such as pollen and dust. When an allergen binds to an IgE antibody on the surface of an immune cell, it triggers the release of histamine and the subsequent reaction that makes the body swell and itch.

Antihistamines prevent this by blocking the receptors that histamine uses to initiate the reaction. Antihistamines can reduce allergic reactions in the following ways: 

  • Blocking H1 receptors at the site of the allergic reaction

Antihistamines reduce the severity of allergic reactions because they block histamine receptors in the body. This is particularly helpful with allergic reactions involving the respiratory tract, such as asthma and rhinitis. These reactions are caused by antibodies called IgE binding to harmless substances such as pollen, dust, and pet dander.

When these substances are inhaled or enter the digestive system, the immune system treats them as invaders and releases histamine. Antihistamines prevent this from happening by blocking the H1 receptors that are responsible for triggering the release of histamine. 

  • Blocking H1 receptors in the central nervous system

Some antihistamines can also cross the blood-brain barrier and block histamine receptors in the central nervous system. This is helpful for allergic reactions when the skin comes into contact with an allergen, such as allergic rashes.

Side Effects of Antihistamines

There are many types of side effects associated with the use of antihistamines. The most common ones are drowsiness and difficulty concentrating. While most antihistamines cause drowsiness, others are less likely to cause this side effect. Other common side effects include dry mouth, blurred vision, a decrease in blood pressure, and a decrease in urination.

While most antihistamines are considered safe during pregnancy and breastfeeding, they have been shown to cause side effects in the fetus, such as hypotonia (low muscle tone). If you are pregnant or breastfeeding, you should consult your doctor before taking an antihistamine.

1. Antihistamines and Pregnancy

Generally, antihistamines are considered safe for pregnant women. However, the fetus experiences some of the side effects of the medication as well. This can cause tremors, respiratory distress, and hypotonia (low muscle tone). If you are thinking about becoming pregnant or are already pregnant, talk to your doctor before taking antihistamines.

Some types of these drugs are considered more dangerous than others. The doctor will most likely recommend that you discontinue using antihistamines, especially during the first trimester, when organ formation is at its highest rate of development. If you are allergic to certain substances and must continue taking antihistamines, be sure to inform your doctor.

2. Antihistamines and Alcohol

Alcohol is a depressant that slows down the central nervous system. This can exacerbate the side effects of antihistamines, such as drowsiness, difficulty concentrating, and blurred vision.

When taken at the same time as antihistamines, alcohol can also increase your blood pressure and heart rate, which can be dangerous to your health. This can also happen when taking antihistamines and caffeinated beverages such as coffee, tea, or cola together.

Conclusion: Understanding Antihistamine

Allergies are a common condition that impacts millions of people. While there is no cure for allergies, antihistamines can treat the symptoms and reduce the severity of allergic reactions. Antihistamines work by blocking histamine receptors in the body. They can be taken either orally or applied topically, and they can also be injected.

While antihistamines are effective at reducing allergic reactions, they do come with side effects. If you have allergies, speak to your doctor about whether or not an antihistamine is right for you.


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