Histamine and Antihistamines
Almost everyone has taken an antihistamine to treat hay fever, itching, to relieve nausea and vomiting, in tablets and syrups used to treat cough and cold symptoms, or as an aid to sleep. The popularity of antihistamines is a mute testimony to the diverse negative effects of histamine. To get a good idea of what histamine can do, let us imagine the effects of an injection of a small amount of histamine:
Histamine symptoms might include:
Headache is felt as a pulsating, whole-head pain, often with a sense of
H1 receptors tend to produce the symptoms already listed and activate allergic reactions.
H2 receptors tend to act as negative feedback receptors and turn the allergic reaction off. They also activate the acid-producing, parietal cells of the stomach lining.
Histamine dilates blood vessels and acts with prostaglandins, PGE2 and PGI2, to produce the early swelling, redness and heat of an inflammatory response. The same mediators may sensitize nerve endings to other pain-producing mediators such as bradykinin. An initial burst of mediator activity will often set a series of cell responses in motion which will amplify and prolong disturbances. Once inflammation is established in tissues by immune cell invasion and mediator release, recovery may take several days to weeks.
Antihistamines are drugs which block the receptors so that the histamine messages are not received. We have drugs that selectively block both kinds of histamine receptors. The common antihistamines (Benadryl, Chlortripalon, Atarax, Claritin, Seldane, and Hismanal) are H1 blockers. The H1 block is useful to treat allergic reactions. The older sedating antihistamines have been used for years and are cheap and effective.
The sedating group act on brain H1 receptors where they cause sedative effects, a dangerous effect if you are driving, operating machinery, or otherwise need to be alert and vigilant. Antihistamine sedative effects are increased by concurrent alcoholic beverage ingestion and by a long list of psychotropic drugs. The sedative effects of some foods such as milk and wheat in susceptible people is not blocked, but enhanced by antihistamines.
The classic antihistamines are represented by chlorpheniramine ( Chlor-Tripalon), brompheniramine (Dimetane), diphenhydramine ( Benadryl) and dimenhydrinate (Gravol). All have been in common use for 40 years with an enviable safety record. These antihistamines have been marketed as allergy preparations for the relief of hay fever and hives and other itchy skin conditions. Antihistamines are included in over-the counter cold and cough preparations even though there is little evidence of benefit and unwanted side effects such as sedation can be risky.
Gravol is marketed as an anti-nausea drug although it is a typical antihistamine. Benadryl has been used as a sedative infants and children, although the occasional child will become restless or hyperactive after taking it. A single bedtime dose of 8.0 mg of chlopheniramine will block allergy symptoms for 24 hours and is often adequate for hay fever relief or relief from skin itching.
The sedating antihistamines are derived from different chemical groups. Here are some examples:
Dimenhydrinate (Gravol) 50-100 mg qid
Unwanted effects are common with these antihistamines, the commonest being sedation, dizziness fatigue, insomnia and dry mouth. Paradoxical hyperactivity occurs in some children. Alcohol increases the sedative effects of these drugs and users are advised to abstain from drinking while on antihistamine therapy. The ability to drive and operate machinery is impaired and should be avoided. Anticholinergic action may cause urinary retention, precipitate glaucoma, and aggravate dementia.
Seldane, Claritin and Hismanal were less likely to cause drowsiness than the older H1 blockers. Seldane and Hismanal ran into big problems with adverse effects.
The following drugs are shown with generic name, trade name, common dose, onset and duration of action.
Terfenadine (Seldane)* 60 mg bd 1-2
hours >12 hours
Over the Counter, generally recognized as safe.
Loratadine (Claritin) 10 mg daily 1-2 hours
* Seldane and Hismanal have been withdrawn from the US market and placed on prescription drug status in Canada because of rare fatal ventricular arrythmias reported with larger than normal doses, in patients with liver disease and when the drugs were administered along with erythromycin, ketoconazole and other drugs. Seldane has been replaced by Fexofenadine ( Allergra), a metabolite of terfenadine with no know cardiac effects. It is non-sedating. Claritin remains a top selling drug.
Other antihistamines and related drugs :
Ketotifen Adult dosage : 1-2 mg bd.
The H2 receptor blocker, cimetidine, first marketed as "Tagamet", joins Valium as one of the best-selling drugs of all time. The H2 block reduces stomach acid secretion. This acid reduction helps to prevent and to heal peptic ulcers. Cimetadine may be thought of as an allergy reaction modifier and antacid combined.
Cimetidine - 400 mg bd
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