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Reviewed by Dr Victoria Lewis, specialist
registrar in dermatology
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What are they used for?
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.
How do they work?
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.
What different types are
there?
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.
Are they suitable for everyone?
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.
When and
for how long do you take them?
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.
What do I need to be aware of while taking
this 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.
Are
there any side effects?
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.
-
Drowsiness.
-
Headache.
-
Blurred vision.
-
Constipation.
-
Dry
mouth.
-
Dizziness.
-
Difficulty passing urine.
-
Confusion.
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.
Can I
take them at the same time as other medicines?
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
-
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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Based on a text by Helen Marshall, pharmacist
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Last updated 10.04.2007
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