By Mr. Suki So (Pharmacist)
Allergic reaction is a part of our body’s defense mechanisms. Histamine, an inflammatory mediator, is released when antigen comes into contact with body cells. It triggers itchiness, redness and swelling, increased secretion (e.g. nasal mucus and tears) and widening of blood vessels (e.g. stuffy nose, red eyes and shortness of breath). Antihistamine can offset the effect of histamine and alleviate allergic symptoms.
Drowsy or non-drowsy medicine?
Some older anti-histamines, such as chlorpheniramine, promethazine* and diphenhydramine*, possess good anti-allergic effect. However, 20-50% of patients would experience tiredness and drowsiness after taking the medicines. Moreover, their effect can only last for 4-6 hours. On the contrary, newer antihistamines, such as loratadine, cetirizine and fexofenadine*, cause less drowsiness (<10%) and their effect can last for 24 hours. All antihistamines cause dry mouth and dry eyes. Patients are therefore recommended to drink enough water.
Decongestants, such as phenylephrine and pseudoephedrine*, do not possess antihistamine property. They constrict blood vessels to relieve stuffy nose. Excitement and increase of blood pressure are the common side effects of decongestants.
Patients often inquire the pros and cons of different antihistamines. In general, patients should pay attention to the following points:
- Choose according to your needs. For example, drivers or salarymen are recommended to use non-drowsy antihistamine. On the other hand, drowsy antihistamine is a good choice for patients in need of more rest.
- Read the instructions carefully or consult a pharmacist. Some non-drowsy medicines do not contain anti-allergic constituents and thus they cannot reduce nasal secretion or itchiness.
- Although combination medicines, which consist of 2-4 medicines, can relieve multiple symptoms, more side effects would be caused. Patients, especially children, are recommended to seek advice from pharmacists for a suitable drug.
The above medicines are nonprescription drugs. Medicines with ‘*’ are sold under pharmacist supervision in a registered pharmacy.
- Micromerdex I Pod version, Truven Health Analytics, Version 1.44.0b1635
- Sweetman SC (ed), Martindale: The Complete Drug Reference. [online] London: Pharmaceutical Press <http://www.medicinescomplete.com/> (Accessed on [14/03/13]).