Antihistamines

Antihistamines are a type of medicine that are often used to treat allergic health conditions that commonly include:

• hayfever,
• allergic skin conditions, such as eczema or urticaria (hives), and
• allergic conjunctivitis (inflammation of the eyes).

Antihistamines work by blocking the effects of a protein called histamine.

What is histamine?

Histamine is a protein that is used by the immune system to help protect the cells of the body from infection.

If your immune system detects the presence of a harmful foreign object, such as bacteria, or a virus, it will release histamine into nearby cells.

The histamine causes small blood vessels to expand and the surrounding skin to swell. This is known as inflammation. The expansion in blood vessels allows an increased number of infection-fighting white blood cells to be sent to the site of the infection, and the swollen skin means that it is a lot harder for an infection to spread through the skin to other parts of the body.

When does histamine need to be blocked?

Usually, histamine is a useful protein but, if you are having an allergic reaction, it is sometimes necessary to block its effects.

Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, as a threat. Your immune system releases histamine, which kick-starts the process of inflammation, causing the skin to become red and swollen.

Histamine also affects the nerves in the skin, which can cause the skin to feel itchy.

Inflammation that occurs inside the lungs can lead to a build-up of mucus and a narrowing of the airways. This  can cause a range of symptoms including:

• sneezing,
• coughing, and
• shortness of breath.

Types of antihistamine

There are a number of different types of antihistamine medicines available, classified as either:

• first generation antihistamines, or
• second generation antihistamines.

First generation antihistamines

First generation antihistamines are the antihistamines that were created in the years following World War Two.

While clinically effective, the one drawback of first generation antihistamines is that they can affect the brain. This can cause a range of possible side effects, the most common of which is drowsiness.

Examples of first generation antihistamines include diphenhydramine and chlorphenamine.

Second generation antihistamines

Second generation antihistamines are antihistamines that were developed during the 1990s. The advantage of these newer antihistamines is that they do not have an effect on the brain, so the range of possible side effects is much smaller.

As second generation antihistamines do not usually cause drowsiness, they are sometimes also known as non-sedating antihistamines.

Examples of second generation antihistamines include loratadine and cetirizine.

What type of antihistamine should be used?

Second generation antihistamines are usually recommended, although there may be some circumstances where the drowsiness that is caused by first generation antihistamines can be of some use. For example, if you are having problems sleeping because you have itchy skin.

Availability

Antihistamines are available in tablet form (oral antihistamines), in the form of creams (topical antihistamines), and as a nasal sprays.

Many antihistamines are available over-the-counter (OTC), which means that you can buy them from your pharmacist without the need for a prescription.

However, although antihistamines are a non-prescription medication, do not assume they are safe for everyone to take.

Before taking antihistamines, always read the information leaflet that comes with the medicine. 

Glossary

Blood

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Immune system

The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.

Blood vessels

Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.

Swelling

Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Sneezing

Sneezing is an involuntary expulsion of air and bacteria from the nose and mouth.

Allergen

An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.

Allergic

An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.

Sickness

Vomiting is when you bring up the contents of your stomach through your mouth.

Stomach

The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.

Ulcers

An ulcer is a sore break in the skin, or on the inside lining of the body.

Drowsiness

Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.

Heart

The heart is a muscular organ that pumps blood around the body.

Fever

A fever is when you have a high body temperature (over 38°C or 100.4°F).