Antihistamines are a type of medicine that's often used to treat a number of allergic health conditions.
These include:
For more information about the conditions that this type of medicine can help treat, see Antihistamines - what it is used for.
Antihistamines work by blocking the effects of a protein called histamine (see below). They're available in tablet or capsule form (oral antihistamines), creams, lotions and gels (topical antihistamines) and as a nasal spray.
How antihistamines work
Histamine is a protein that the immune system uses to help protect the body’s cells against infection. The immune system is the body’s natural defence against illness and infection.
If the immune system detects 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 of the blood vessels allows an increased number of infection-fighting white blood cells to be sent to the site of the infection. The swelling of the surrounding skin also makes it harder for an infection to spread to other parts of the body.
Histamine is usually a useful protein, but if you're having an allergic reaction it's sometimes necessary to block its effects. Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, as a threat.
The release of histamine causes the process of inflammation to begin and leads to nearby tissue becoming red and swollen. It can also affect the nerves in the skin, making the skin feel itchy.
Types of antihistamine
There are a number of antihistamine medicines, which are classified in two groups. These are:
- first-generation antihistamines, which cause symptoms of drowsiness in most people; they include diphenhydramine and chlorphenamine
- second-generation antihistamines, which do not usually causes symptoms of drowsiness and include loratadine and cetirizine
Second-generation antihistamines are usually recommended. Do not underestimate the levels of drowsiness caused by first-generation antihistamines – their effects can continue into the next day if you only take them at night.
Research has found that adults who regularly take first-generation antihistamines are more likely to be involved in serious accidents. Similarly, children who regularly take first-generation antihistamines perform less well at school than would usually be expected.
An exception to these recommendations is sometimes made if the drowsiness caused by first-generation antihistamines can be of some use. For example, if you're having problems sleeping because you have itchy skin.
Where do I get them?
Many antihistamines are available direct from your pharmacist without prescription.
Safety
Even though antihistamines are a non-prescription medication, you shouldn't assume that they're safe for everyone to take.
It's also important to only take antihistamines as directed. Overdoses are possible and overuse can lead to addiction.
Before taking antihistamines, always read the patient information leaflet that comes with the medicine.
For more information see Antihistamines - who can use it.
