Histamine is an enzyme, occurring naturally in the body in low amounts. It is secreted normally by many cells of the human body. Some of these cells include: cells of the immune system (mast cells and basophils), stomach and cells of central nervous system. It plays a vital part in our bodies, defending us against infections, controlling our appetite and wakefulness and controlling our blood pressure. It works by binding to specific, designated areas, called histamine receptors, located throughout the human body. (e.g. the brain, blood vessels, airways and bone marrow). When there is too much histamine circulating in the blood stream, unwanted histaminergic effects occur, and the normal function of histamine is no longer considered physiological.
In allergic conditions (e.g. hay fever, allergic conjunctivitis, atopic dermatitis, asthma and anaphylaxis), histamine is released in abundance (above physiological values) and serious histaminergic effects can occur. These effects include: sneezing, itching, runny or blocked nose, itching, red and teary eyes, abdominal pains and cramps, itching and flushing of the skin, headache, a fast heartbeat, dizziness, irritability and poor concentration, drowsiness and in severe cases, breathing difficulties or low blood pressure and shock (anaphylaxis). If not treated early and effectively, anaphylaxis can be fatal.
If you or your child suffers from allergies, you will probably know the name antihistamines. These drugs, available as syrups, tablets, nasal sprays, eye drops and intravenous formulations, compete with histamine at receptor sites and attempt to reduce the action of histamine. It is therefore not surprising that antihistamines are the most common drug prescribed in patients with allergic conditions and it should be given early in any patient suffering from an anaphylactic event (not forgetting of course the potential life-saving effect of adrenaline in this situation as a priority).
Are all antihistamines the same and are they safe?
Although all antihistamines oppose the action of histamine, they differ quite significantly in their chemical structure, leading to a huge difference in especially their duration of action, side effect profile and the ability to interact with other drugs taken simultaneously.
Antihistamines can conveniently be subdivided into two groups: older (first generation-) antihistamines and newer (second generation-) antihistamines. Some examples and the main difference in side effects of commonly prescribed antihistamines, are summarized in the table below (not all generics are named- there are many available):
Are there safe doses for antihistamines?
New generation antihistamines are regarded as safe. The central nervous system side effects of the first-generation antihistamines make them less popular, especially in children. It should be noted, that even in normal dosing, second-generation antihistamines can cause drowsiness in some individuals.
Careful attention should be paid to the to the correct dose and dose interval of all antihistamines. Taking antihistamines on a “full” or “empty” stomach will not generally influence their efficacy and side effect profile.
The following table summarizes the ‘safe’ doses of most antihistamines available in South Africa:
The choice of an appropriate antihistamine: which one to whom?
Your doctor or pharmacist should take a good medical history from you, before prescribing any antihistamine. An educated choice of an appropriate antihistamine and the appropriate dose will be influenced by the following: pregnancy and breastfeeding, underlying diseases, young and old age, simultaneous use of other medication, affordability of the antihistamine and your occupation.
In pregnancy and breastfeeding, the use of any first-generation antihistamine, are considered unsafe, as these can affect the fetus and are excreted in breastmilk. In these circumstances, cetirizine and loratadine are considered relatively safe.
When taking other medication (e.g. rifampicin, erythromycin, azithromycin, antifungals, antimalaria or some migraine medication) simultaneously, it is safer to use fexofenadine. It should be noted that simultaneous intake of grapefruit juice, may cause decreased blood levels of fexofenadine, leading to the drug being less efficacious.
Toxic side effects, especially cardiac toxicity, are concerns when cetirizine, levocetirizine, desloratadine and fexofenadine are taken with grapefruit juice – blood levels of these antihistamines may be increased.
Due to their low side effect profile in the brain, fexofenadine and loratadine are generally regarded as safe for patients involved in military operations and for patients practicing a so called “critical occupation” e.g. commercial airline pilots.
In the elderly, dose adjustments (using lower doses) of second-generation antihistamines should be considered, due to liver and renal functions normally decreasing with age. First generation antihistamines, should not be used in the elderly and careful consideration of underlying heart and other diseases of the elderly are mandatory.
Treating the itching in atopic dermatitis, can be problematic, especially at night. The second-generation antihistamines have been shown to be less effective, while first generation antihistamines are prescribed for their sedative effect, rather than their anti-itch properties.
Does the price of antihistamines play a role in their efficacy?
While second generation antihistamines are more expensive that first generation antihistamines, their cost does not outweigh the potential serious and life-threatening side effects of the first-generation antihistamines. Taking a second-generation antihistamine is therefore always advisable. In general, all second-generation antihistamines are effective in treating allergies and paying more for an antihistamine does not necessarily mean it will be more effective. Taking your antihistamine before your exposure to a known allergen (the substance that you are allergic to) will make your antihistamine to be more effective. Careful allergen avoidance strategies, together with an appropriate antihistamine, will ensure treatment success in most patients suffering from allergies.
Can I/ my child get addicted to antihistamines or develop tolerance to it?
No, there are no addiction risks to antihistamines!
When evaluating the development of tolerance to an antihistamine, recent studies indicate that this does not exist. If your allergy symptoms are getting worse while on an antihistamine, is probably due to the natural course of your allergic condition rather than your antihistamine becoming less effective. The level of allergens (e.g. pollen) that you are exposed to, may also differ from season to season, causing your symptoms to worsen periodically – even if you are on an antihistamine. Another major cause of deterioration is an intercurrent viral cold, which can “mimic” allergies.
When is it the optimal time to start using my antihistamine? Should I take my antihistamine the whole year around or can I stop it when I’m well or when the pollen season is over?
Persistent allergic rhinitis tends to occur all year around, with more problematic symptoms occurring during certain periods of the year. It is therefore advisable to use a safe, non-sedating antihistamine in the correct dose, all year around in the case of persistent symptoms to all-year allergens such as house dust mite. Speak to you doctor or pharmacist about swopping to another second-generation antihistamine, if you experience drowsiness or poor concentration while using your current antihistamine.
If you have typical seasonal symptoms, such as grass pollen allergy (typical hayfever) , you may not need to use your antihistamine throughout the year, it is important to remember that pollen counts differ from year to year and the use of pollen calendars may certainly be helpful. As mentioned before, antihistamines work better when taken well in advance of your allergen exposure or before the pollen season starts. Knowing your geographical area’s pollen count during certain periods, will certainly help you to anticipate your “problem” times of the year.
Remember that other treatments may need to be taken in conjunction with your antihistamine for optimal symptom control, for example steroid nasal sprays are very important in rhinitis management, and antihistamines are not an appropriate sole treatment for asthma or eczema.
General comments and conclusion
Second generation antihistamines are generally safe and effective, and their use is preferred above the older, first generation antihistamines. All antihistamines should be taken after a thorough consultation with your doctor or pharmacist. Careful consideration of pre-existing medical conditions, the simultaneous use of other drugs, pregnancy and breastfeeding and age of the patient should guide the healthcare practitioner in choosing an appropriate antihistamine. Antihistamines work better when taken before the allergy season starts or before contact with a known allergen. There is no risk of becoming addicted or tolerant to your antihistamine.