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Sunday April 16, 2006

Antihistamines made simple

ANTIHISTAMINES work by blocking the action of histamine at special sites (receptors) in the skin, nose, blood vessels and airways.  

Histamine is a natural chemical produced by the body in response to foreign organisms. It is an important part of the body’s defence mechanism.  

But sometimes the body releases too much histamine and this produces the allergic reaction. 

For example, in hay fever, too much histamine is released in response to pollen and the histamine receptors in the nose are overstimulated, which irritates the nose and eyes, causing sneezing, an itchy, runny nose and red, itchy eyes. 

Antihistamines can help treat the symptoms of allergy.

What types of over-the-counter (OTC) antihistamines are available?  

OTC drugs are medicines you can buy without a doctor’s prescription. Two types of OTC antihistamines are available: first generation antihistamines such as promethazine and chlorpheniramine; and second generation antihistamines that include loratadine and cetirizine. 

Note: Both types of antihistamines are often mixed with other drugs such as pain relievers or decongestants.  


What symptoms can OTC antihistamines treat? 

Antihistamines are used to relieve the symptoms of allergies like hay fever (seasonal rhinitis) and perennial rhinitis (hay fever-like symptoms all year round), certain allergic rashes such as urticaria (nettle rash), itchy skin (pruritis), insect bites and stings. These symptoms include sneezing, itchy and watery eyes, and a runny nose.  

Antihistamines can also relieve itchiness caused by insect bites and stings.  

Some antihistamines are used to prevent motion sickness. Because one of the most common side-effects of antihistamines is feeling sleepy, they are sometimes used to help people who have insomnia. 


What are some common side-effects of OTC antihistamines? 

Side-effects from antihistamines aren’t common for healthy adults. However, side-effects can be a concern for older adults or people who have health problems. 

First-generation antihistamines such as promethazine and chlorpheniramine can make you feel very sleepy.  

This can affect your ability to drive or operate machines. It can also make it hard for you to think clearly.  

Antihistamines can cause your mouth and eyes to feel dry. Second-generation antihistamines such as loratadine and cetirizine are not as likely to cause these side-effects. 

Other less common side-effects, from the sedating antihistamines in particular, are headache, difficulty in passing urine, dry mouth, blurred vision, and digestive tract problems such as feeling or being sick, constipation or diarrhoea.  

Rarely, some antihistamines can also cause palpitations and abnormal heart rhythms, high blood pressure, allergic reactions (such as swelling, rash, and breathing difficulties), dizziness, confusion, depression, disturbed sleep, tremor, convulsions, blood and liver disorders, and over-excitement in children.  

Children and older people are more likely to experience side-effects.  

Antihistamine preparations applied locally (i.e. to the nose, eyes or skin) may cause irritation.  


Could OTC antihistamines cause problems with any other  

medicines I take? 

Yes. Antihistamines can interact with other drugs you take. If certain drugs are taken at the same time, they can interact with each other and change the way your body processes them. This is called a drug interaction. When this happens, the risk of side-effects increases. 

Antihistamines are often combined with decongestants and/or pain relievers.  

If you take one of these combination medicines, it’s important to understand each of the active ingredients and the interactions they may have with other drugs you’re taking. 

Be sure not to “double up” on antihistamine. Many OTC cold and allergy products contain antihistamines, as do some prescription drugs.  

If you take more than one of these products, you can take much more antihistamine than you intend. 


Should I avoid any foods, drinks or activities while taking antihistamines? 

Alcohol can increase the drowsiness caused by antihistamines. Also, be very careful if you drive a car or run machines while taking an antihistamine. Antihistamines may slow your reactions without you even being aware of it. 


Who shouldn’t take antihistamines? 

Talk to your doctor before using a first-generation antihistamine if you have any of the following health problems: 


·Trouble urinating (from an enlarged prostate gland) 

·Breathing problems, such as asthma, emphysema or chronic bronchitis 

·Thyroid disease 

·Heart disease 

·High blood pressure 

Before taking a second-generation antihistamine, tell your doctor if you’ve ever had kidney or liver disease. 


What should I look for on the drug label? 

When choosing an OTC antihistamine, check the drug label for possible side-effects or interactions with other drugs you’re taking. This information will appear in the “Warnings” section of the label. 

Be sure to check that you’re not taking two medicines that contain the same ingredient. You will find this information in the “active ingredient” section. 

You also need to get advice if you are pregnant or breastfeeding because some antihistamines should be avoided in pregnancy and maypass into breast milk. 


Simple non-drug measures for allergies 

The best way to control an allergy is by finding out what you are allergic to and then avoiding it. This can be possible if it is something obvious that you’ve touched (like nickel in jewellery) or eaten (like shellfish).  

If you have hay fever though, it can be more difficult to avoid pollen. Stay indoors, with the windows shut, as much as possible when there is a high pollen count and avoid newly mown lawns or fields.  

When you have to go outside, sunglasses can help stop pollen getting in your eyes, and washing your face and hands when you come in can help. 



1) British National Formulary (BNF) 51 

2) MIMS 102nd Edition 2005 

3) British United Provident Association, BUPA  


n Diong Swee Hoon is a pharmacist. For more information, e-mail The information provided is for educational purposes only and should not be considered as medical advice. The Star does not give any warranty on accuracy, completeness, functionality, usefulness or other assurances as to the content appearing in this column. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.