How Antihistamines Work For Allergies
Many people find the worst symptom of their eczema is severe itching. Scratching relieves the itch, but also makes the inflammation and the itching worse because it causes more inflammatory substances to be released within the skin. This is often referred to as an ‘itch-scratch-itch’ cycle. It leads to thickening and weeping of the skin and generally makes the eczema worse and more likely to become infected because scratching breaks the skin. Antihistamines are medicines that can be used to relieve severe itching and help break this cycle.
Often you will find that the itchiness of your skin is reduced when you use regular moisturisers to keep the skin soothed and hydrated, and control the inflammation with topical corticosteroids or other newer medicines. However, if itching is still a problem your doctor may prescribe you an antihistamine.
Antihistamines are most commonly used to control the symptoms of allergies such as hayfever. In these conditions they work by preventing the actions of histamine, which is a substance produced by the body as part of its natural defences. It is stored in cells called mast cells, in almost all tissues of the body, and is released when the body reacts to a foreign substance (known as an allergen).
The released histamine binds to its receptors (H-1 receptors) causing a chain reaction that includes an increase in blood flow to the area, and the release of other chemicals that add to the allergic response. Itching is one of the results.
Antihistamines work by blocking histamine receptors, therefore reducing the reactions that cause itching. However, histamine is only one of many substances in the body that cause itching, and these medicines are mainly of value because they cause sedation.
There are two different types of antihistamine – the older group, sedating antihistamines, can enter the brain and cause drowsiness, while the newer non-sedating antihistamines do not.
Itching can occur during the day but is usually worse in early evening and at night, so sedating antihistamines are used because they help make you sleepy, as well as reducing the itch. The most commonly used ones are promethazine (Phenergan), hydroxyzine (Atarax), or alimemazine (Vallergan).
Non-sedating antihistamines such as cetirizine or loratadine are not effective at controlling the itch of eczema. Antihistamines are also available as creams, but these too are ineffective at reducing the itchiness of eczema and can cause allergic reactions in the skin.
Antihistamines may cause side effects that could worsen conditions such as glaucoma, enlarged prostate gland, retention of urine, or obstruction of the gut, and for this reason they should be used with caution in these conditions. In addition, if you have liver or kidney disease you may need a lower dose of these medicines, though this depends on the particular medicine prescribed.
Medicines that are not totally essential should ideally be avoided during pregnancy, and pregnant women should not take antihistamines unless prescribed by a doctor. The safety of antihistamines during pregnancy has not been fully established and they should only be used if the benefits outweigh any possible risks to the baby.
Antihistamines can be used in children though age limits do apply: hydroxyzine is not recommended for children under six months, and promethazine and alimemazine are not recommended for children under two years.
Sedating antihistamines will usually be prescribed to be taken at bedtime and are usually only used for short periods of time, for example when a severe flare-up is causing intense itching.
kind of medicine
Sedating antihistamines cause drowsiness and may therefore affect your ability to drive or operate machinery safely. This should not be a problem if you take them before going to bed, but be aware of your potentially reduced ability if you are going to perform any hazardous tasks.
Drinking alcohol with a sedating antihistamine can increase the drowsiness it causes and should be avoided.
All medicines have possible side effects, though you may not necessarily experience any. Side effects from antihistamines are more likely to occur in children and the elderly, and some of the most common ones associated with this type of medicine are listed below.
- Blurred vision.
- Dry mouth.
- Difficulty passing urine.
More information about the potential side effects associated with each specific medicine can be found in the patient information leaflet that will be provided with the medicine.
It is important to tell your doctor or pharmacist what medicines you are taking, including those bought without a prescription and herbal medicines, before you start taking a new medicine.
Since sedating antihistamines cause drowsiness there is an increased risk of drowsiness if they are taken with any of the following medicines, which can also cause drowsiness:
- tricyclic antidepressants, eg amitriptyline
- strong painkillers containing opioids, eg morphine, codeine, dihydrocodeine
- benzodiazepines, eg diazepam, temazepam, lorazepam
- other sedating antihistamines, eg chlorphenamine
- sleeping tablets, eg zopiclone.
There may be an increased chance of side effects such as dry mouth and constipation if sedating antihistamines are taken with other medicines that can have these types of side effects, including:
- antispasmodic medicines, eg hyoscine
- other antihistamines, eg meclozine
- anticholinergic medicines for Parkinson’s disease, eg procyclidine
- anticholinergic medicines for urinary incontinence, eg oxybutynin, tolterodine
- certain antidepressants
- antipsychotic medicines.
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