What is histamine?
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:
- Insect bites
- Urticaria (hives), angioedema and anaphylaxis
- Scombroid fish poisoning
- Urticaria pigmentosa.
What are antihistamines?
Antihistamines are medications that counteract the effect of the natural chemical histamine.
What are antihistamines used for?
Antihistamines are used to reduce symptoms due to type 1 allergic diseases (such as anaphylaxis) and other conditions mediated by histamine, such as urticaria. They do not always result in complete control because they do not counteract the 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 antagonists. Antihistamines used in dermatology are mainly H1 antagonists.
As well as tablets, antihistamines are available as injections, elixirs, and creams.
- An antihistamine 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 to induce sleep, they may be prescribed in other itchy skin diseases such as eczema. They are not thought to be addictive but cause sedation, respiratory depression and cognitive impairment.
Other uses of antihistamines
Antihistamines are also used for:
- Sedation, for example before an operation
- Reduce nasal secretions in hay fever or a cold
- Cough suppressant
- Counteract motion sickness
- Increase appetite in those who are unwell.
Conventional or 'sedating' H1 antihistamines are obtainable in New Zealand without prescription. In December 2018 they include:
- Pheniramine (as eye drops for allergic conjunctivitis)
- Chlorpheniramine maleate /chlorphenamine (Histafen™; and several combination products marketed for upper respiratory tract infections)
- Dexchlorpheniramine maleate (Polaramine™).
Longer-acting phenothiazines (these can help reduce nausea but can be quite sedating)
- Promethazine (Phenergan™, Allersoothe™)
- Trimeprazine or alimemazine (Vallergan™).
- Diphenhydramine (Unisom Sleepgels™; in combination Benadryl™ for a cough, Panadol Night Caplet™ and others)
- Doxylamine (Dozile™)
- Triprolidine, available in combination (Actifed™, Codral™ Daytime/Nighttime Tablets)
- Cyclizine, meclozine (Marzine™), often used for travel sickness.
- Azatadine (Zadine™)
- Hydroxyzine (Serecid™)
- Cyproheptadine (Periactin™), which also has antiserotonin effects (useful for a migraine and to increase appetite).
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 a cough and cold symptoms, and they should not be used for insomnia in children under the age of 12 years. Side effects might be greater in older patients. At any age, the manufacturer's recommended dose should not be exceeded without medical advice.
Side effects are anticholinergic (antimuscarinic) and include:
- Drowsiness and slowed reaction times affecting the performance of skilled tasks
- Dry mouth, blurring of vision, difficulty passing urine and impotence (antimuscarinic effects)
- Paradoxical stimulant effect (wakefulness and even agitation)
- Cardiovascular side effects such as irregular heartbeat in higher doses
- Cognitive impairment and possibly, dementia.
Alcohol in combination with antihistamines 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 underwater.
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 bloodstream.
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.
- Mepyramine (Antisan™ cream), an ethylenediamine.
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 and anticholinergic effects but can do so in susceptible individuals. The dose should be reduced in renal impairment and in older people.
Those available in New Zealand (December 2018) are:
- Cetirizine hydrochloride (Zyrtec™, Razene™, Zetop™, Allerid™, Zista™, Histaclear™ and others) and levocetirizine (Levrix™, Levoclear™)
- Loratadine (Claratyne™, Lora-Tabs™, Loraclear™, LoraPaed™, Clarinase™, Lorfast™ and others) and desloratadine (Claramax™, Aerius™, Deslor™)
- Fexofenadine (Telfast™, Fexofast™, Fexaclear™, Hayfexo™, Xergic™).
Non-sedating antihistamines include rupatidine, which is similar to loratadine. It is not yet available in New Zealand (December 2018).
Terfenadine (Teldane™) and astemizole (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:
The 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 been used for some patients with urticaria but are no longer recommended for this indication.
H2 blockers available in New Zealand include:
- Ranitidine (Zantac™, Peptisoothe™)
- Famotidine (Pepzan™).
Side effects of H2 blockers are uncommon but include:
- Gastrointestinal upset (diarrhoea)
- Hair thinning
- Gynaecomastia (breast enlargement in males)
- Confusion, psychosis and possibly dementia due to weak antimuscarinic effects
- Reversible impotence
- Blood count, liver, kidney, pancreatic disturbance (very rare).
If you are not based in New Zealand, we suggest you refer to your national drug approval agency for further information about medicines (eg, the Australian Therapeutic Goods Administration and the US Food and Drug Administration) or a national or state-approved formulary (eg, the New Zealand Formulary and New Zealand Formulary for Children and the British National Formulary and British National Formulary for Children).