Antihistamines and Allergy
Antihistamines and Allergy
Antihistamines were first introduced in 1947, and are very widely used, so their safety — at least in the case of the older antihistamines — is beyond doubt. Most of the antihistamines have no major ill effects, and no one should feel concerned about taking them. At worst they produce some rather annoying minor side effects, such as drowsiness, which often wear off in time.
These drugs are particularly valuable for hayfever and other allergies in the nose (perennial allergic rhinitis). They are also used for chronic urticaria, sometimes in combination with anotherhistamine-blocking drug — see p. 53.
Antihistamines are not much used for asthma. They have relatively little effect, probably because so many other messenger chemicals are involved in an asthma attack. However, doctors in Japan do use antihistamines for asthma, and it is possible that people of Asiatic origin react differently to them.
Only one antihistamine, ketotifen, is widely used for asthma in the West, and this has other effects besides blocking histamine (see p. 159). A new role may soon develop for antihistamines in thetreatment of asthma, combined with anti-leukotriene drugs (see p. 159).
If you suffer from anaphylaxis you might be given antihistamines in a liquid or chewable form, for use in an emergency. These are not enough in themselves to treat this dangerous condition - you must have an adrenaline injector (see p. 150).
In the past, some doctors prescribed antihistamines for atopic eczema, mainly for their sedative effect(see p. 139) which was thought to help children to sleep better and scratch less at night. This treatment has largely gone out of favour, because its value is in doubt. But a recent study has revealed that the non-sedating antihistamine cetirizine may be useful for very young children with atopic eczema, not only in treating their skin, but also in reducing the chance of them developing asthma (see p. 249).
Most people take their antihistamines in tablet or capsule form. Syrups and sugar-free elixirs areavailable for children.
Antihistamines can also be applied directly, in the form of nasal sprays or eye drops. These are mainlyused to treat hayfever and the conjunctivitis (inflammation of the eye) which often accompanies it.Levocabastine (brand name Livostin) is particularly effective for the eyes.
Antihistamine creams are also sold, without prescription, for the treatment of insect bites - i.e. thenormal non-allergic reaction to such bites. These creams are not recommended for atopic eczema or otherallergic conditions affecting the skin. Not only are they unlikely to help, but they may make mattersworse because, with regular use, skin sensitisation to the antihistamine occurs very readily (see pp.54-5).
Some common brand names
Common brand names include: non-sedating antihistamines - Clarityn, Semprex, Zirtek; Mistamine, Mizollen, Telfast, Terfenadine. Thefirst three are available without prescription.
older (sedating) antihistamines — Atarax, Dimotane, Optimine, Periactin, Piriton, Tavegil, Vallergan eye drops — Emadine, Livostin, Optilast nasal sprays — Livostin, Rhinolast
How antihistamines work
Of the messenger chemicals released when an allergic reaction occurs, the most important is histamine.
This does its work by attaching to specialised receptors in certain parts of the body, and so
triggering various reactions (see box on p. 12). The action of antihistamines is very simple: they bind
to the same receptors as histamine, but they do not trigger any reaction. Histamine cannot bind to the
receptor because the antihistamine is already there.
Unfortunately, the reverse is also true: if the histamine is already there, the antihistamine cannot
elbow it off the receptor, which is why it is important to take the antihistamine well before the
allergen is encountered. Taking antihistamines at the first sign of a snuffle or itch can also work,
but the effects will not be nearly as good as taking them in anticipation of an exposure.
The best approach to treating hayfever, for example, is to start taking the antihistamines at least a
week before the pollen season begins, and preferably two to three weeks before. You should then take
them continuously until it is over. This will make a huge difference to the degree of symptom control
you achieve.
Side effects
The older types of antihistamine, such as chlorphenamine (brand name, Rriton) are relatively
non-specific in their effects – they bind to several different kinds of receptors, not just those for
histamine. As a result they can have some unwanted effects, such as causing drowsiness and poor
coordination. While these sedative effects are no cause for concern in themselves, they can, of course,
be hazardous if you work with dangerous machinery or drive. Avoid both until you are sure how you react
to the antihistamine. Note that the effects of alcohol may be increased.
Very occasionally antihistamines have the opposite effect, causing stimulation rather than sedation;
this is most likely to occur in children and old people. Lowering the dose may solve the problem.
The other possible side effects of the older antihistamines –all of which are minor ones – are
headache, dry mouth, blurred vision, difficulty in passing urine, nervousness, shaky hands, upset
stomach or diarrhoea. A few men suffer impotence while taking antihistamines, but this disappears when
the drug is stopped.
The minor side effects of antihistamines, including drowsiness, often wear off after a while, although
the benefits of the drug remain. So it is worthwhile persisting with an antihistamine, even if it
causes some problems at first. Many people experience side effects from certain antihistamines but not
from others, so try several different types to find one that suits you.
The problem of drowsiness has been reduced, in recent years, thanks to the development of new drugs
that are far more
specific for histamine receptors, the non-sedating antihistamines. A few people do get drowsy even with
these drugs. Again, the effects vary from one drug to another, so if the first one disagrees with you,
try a different one.
It is worth noting – since some people may still have the odd packet in their medicine cabinet – that
two of the non-sedating antihistamines that were available without prescription a few years ago proved
to be unsafe for a small minority of people. One was astemizole (brand names: Hismanal, Pollon-eze),
which has now been withdrawn from use altogether in Britain. The other was terfenadine (brand names:
Triludan, Seldane, Terfenadine) which is still available, but only on prescription.
There are several special precautions relating to terfenadine:
• Never exceed the correct dose.
• If you have ever had any kind of heart problem, talk to your doctor before taking terfenadine.
• Stop taking the drug if you have palpitations, or if you feel faint; see your doctor promptly.
• Do not take terfenadine if you are taking the antibiotic erythromycin, or anti-fungal drugs
such as ketoconazole (Nizoral) or fluconazole (Diflucan), used to treat vaginal thrush.
• Do not take terfenadine if you have liver disease.
• Do not drink grapefruit juice while taking terfenadine: something found naturally in grapefruit
interacts unpleasantly with this antihistamine.
In addition to these special precautions concerning terfenadine, any antihistamine should be treated
with caution by those suffering from epilepsy, Parkinson’s disease, glaucoma, prostate enlargement,
kidney problems, urinary retention, a gastric ulcer, a thyroid disorder, porphyria or liver disease.
Check with your doctor before taking antihistamines if you have any of these conditions.
It may be inadvisable to use antihistamines if you are taking sleeping tablets, anti-depressants or
anti-anxiety drugs – again, see your doctor.
Stop taking antihistamines if you suffer any unusual kind of rash, or if your skin becomes more
sensitive to sunlight.
If you are breast-feeding, note that, because they go through into the milk, the older antihistamines
may make the baby sleepy. However, they do no harm.
Rescue treatment
Most antihistamines perform very badly if you take them once the allergic reaction has set in, but
acrivastine (Semprex) can be good in these circumstances and is non-sedating. No prescription is
required for this drug.
possibly identify all major side effects. We vary in our response to drugs, because we are all so
different at the chemical and cellular level. A drug might have a serious side effect that only affects
one person in 10,000, and no safety trial can hope to identify such a rare response. Only when a drug
is released, and becomes widely used, do such side effects come to light. Other unanticipated side
effects can sometimes arise when people taking the new drug are much older than those in the safety
trials, or belong to a different ethnic group with different susceptibilities. Combining the drug with
certain other drugs can also be a potential source of trouble, although pharmaceutical experts can
often predict such problems from a detailed knowledge of the chemistry of drugs and how they are broken
down in the body. Side effects that take several years to develop - more than the timespan of most
safety trials - will also fail to show up until the drug has been released.
All this may sound very alarming, but in fact severe reactions to new drugs are not that common. And
there are various safety nets in place - doctors keep a close eye on patients taking new drugs, and a
special reporting system ensures that, if unexpected side effects do show up, the information is
quickly shared with others in the medical community.
In order to relate the information here to a particular medicine that you take, you need to know what
drug category it belongs to. Does your inhaler contain a beta-2 reliever, a steroid, a cromoglycatetype
drug or an anti-cholinergic, for example? If you are not sure, ask your pharmacist.
Those are the category names for drugs: they denote families of drugs which are similar chemically
and work in roughly the same way. Within each category, or family, there are a number of individual
drugs. The individual drugs should, ideally, have a standard internationally agreed name - this is
known as the generic name. Unfortunately, a few of the drugs used for allergies and asthma have more
than one generic name - salbutamol is known as albuterol in some parts of the world, and adrenaline is called epinephrine.
Finally there are the brand names, which are the ones most patients are familiar with. These are always
shown with a capital letter, unlike the generic names. Long-established drugs are usually made by
several different pharmaceutical companies, and therefore marketed under several different brand names.
A newer drug, which is still covered by the patent of the pharmaceutical company that developed it,
will be sold under only one brand name.
The issue of brand names is important, because a different brand name might make you think you are taking a different drug, when in fact it is exactly the same drug being marketed in a different guise.If you have suffered side effects from a particular drug in the past, and wish to avoid it in future, take note of its generic name, rather than its brand name. Sometimes the generic name is used as the brand name, in what are called generic drugs. These arerelatively inexpensive copies of popular drug brands -they are just the same chemically, but they costless because there is no advertising of the brand to doctors, and profit margins have been cut to aminimum. In order to reduce National Health Service costs, doctors are now asked to prescribe generic drugs whenever possible.