Author: Amanda Oakley MBChB FRACP DipHealInf, Dermatologist, Waikato Hospital, Hamilton, New Zealand, 2001.
Histamine is a vasoactive chemical. This means it has an effect on small blood vessels, resulting in dilated capillaries (redness), and leakage of protein-rich fluid into surrounding skin (swelling). Histamine affects nearby nerves resulting in itching. Histamine is stored in granules within mast cells together with other vasoactive chemicals.
Histamine release occurs in the following skin conditions:
Antihistamines are medications that counteract the effect of the natural chemical histamine. They are valuable drugs to reduce symptoms due to allergic diseases, especially those mediated by histamine, such as urticaria. They do not always completely control the allergic reaction because they do not counteract other chemicals that may be responsible for the symptoms.
There are at least two kinds of histamine receptors, hence antihistamines are classified as H1 and H2 blockers. Antihistamines used in dermatology are mainly H1 blockers.
As well as tablets, antihistamines are available as injections, elixirs, and creams.
An injection can be given in case of severe allergic reaction (although in such cases, adrenaline (epinephrine) may be more appropriate and can be life-saving).
Topical preparations (ointments and creams) are often applied to insect bites but are not very effective as the antihistamine chemical does not penetrate the skin well.
As the older type of antihistamines result in relaxation and help sleeping, they may be prescribed in other itchy skin diseases such as eczema, particularly taken at night. They are not thought to be addictive, but cause sedation and respiratory depression.
Other uses of antihistamines include:
Conventional antihistamines are obtainable in New Zealand without prescription. In September 2005 they include:
Short acting alkylamines
Longer acting phenothiazines (these can help reduce nausea but can be quite sedating)
In New Zealand, label statements indicate that sedating antihistamines should not be used in children under the age of 2 years for any reason; they should not be used in children under 6 years for cough and cold symptoms; and they should not be used for insomnia in children under the age of 12 years. At any age, the manufacturer's recommended dose should not be exceeded without medical advice.
Side effects are anticholinergic and include:
It is best not to drink alcohol when taking antihistamines, because this combination could result in excessive and potentially dangerous drowsiness. Care should be taken when combining with other sedative or anticholinergic drugs and when driving, flying or diving under water.
Because they are broken down by the liver, those with liver disease are best to take a reduced dose as there will be excessive levels circulating in the blood stream.
In general, all drugs are best avoided in pregnancy, but there is no data to suggest that antihistamines are harmful to the mother or fetus so they are sometimes prescribed for severe urticaria in pregnant women. If a mother is breastfeeding, antihistamines may reduce milk production and could result in drowsiness of the infant, so are best avoided.
Unfortunately, antihistamine creams can occasionally cause contact allergic dermatitis so they should not be applied to large areas, and should be discontinued if they result in a rash. They should be avoided on broken skin.
Non-sedating antihistamines are much less likely to cause drowsiness.
Those available in New Zealand (September 2005) are:
New antihistamines include rupatidine, which is similar to loratadine. It is not yet available in New Zealand (May 2016).
Terfenadine (Teldane™) and astemisole (Hismanal™) have been withdrawn from the New Zealand market. These drugs may result in electrical disturbances in the heart resulting in palpitations and rarely sudden death when taken at the same time as the following medicines:
A second type of histamine receptor (H2) is found in the stomach and skin. H2 blockers were developed to reduce stomach acid secretion and proved useful for patients with peptic ulcers. H2 blockers have also been found helpful for some patients with urticaria.
H2 blockers available in New Zealand include:
Side effects of H2 blockers are uncommon but include:
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