Health A-Z



Antihistamines are mainly used to treat allergies such as hay fever, hives or urticaria, and itching. They may be used to help reduce feeling sick (nausea) and being sick (vomiting).

How do antihistamines work?

Allergic symptoms occur when your body wrongly recognises a food or something (such as pollen spores) in your environment as a threat and sends repair chemicals to deal with these perceived intruders. One of these repair chemicals, histamine, is released from repair cells called mast cells, which are scattered throughout the body.

This histamine can then bind with receptors to trigger increased blood flow to the surrounding area, which can lead to symptoms such as swelling and increased secretions, resulting in a blocked or a runny nose, watery eyes and, most importantly, itchiness.

Antihistamines don’t stop allergic reactions from happening, but they do block the histamine receptors from being able to be triggered by the histamine that is released, reducing your symptoms.

Which antihistamines are available in New Zealand?

Antihistamines come in different forms, including tablets, nasal sprays, eye drops and syrups. There are many brands available on prescription from your GP or over the counter at your local pharmacy. Different antihistamines are better at treating different symptoms, so ask your GP or pharmacist to advise you on which antihistamine is best for your needs. Generally antihistamines are classified into 2 main groups:

Sedating antihistamines Non-sedating antihistamines
  • These can make you feel quite drowsy or sleepy.
  • They are used when the effect of drowsiness is helpful to the condition being treated such as in some skin conditions where itch can cause sleep disturbance.
  • Sedating antihistamines may affect your concentration and performance of some tasks that require you to be alert, such as driving, and operating machinery.
  • These are less likely to cause drowsiness.
  • Although drowsiness is rare, it can occur and may affect performance of skilled tasks such as driving.
  • Take care until you know how these medicines affect you.

Examples available in New Zealand include:

  • chlorphenamine (Histafen®)

  • dexchlorpheniramine (Polaramine®)

  • doxylamine (Dozile®)

  • promethazine (Phenergan®, Allersoothe®) 

Examples available in New Zealand include:

How long do antihistamines take to work?

Usually antihistamine tablets start to work within 30 minutes after being taken, and tend to be most effective within 1-2 hours after being taken.

  • Antihistamines are more effective when taken regularly as a prevention, before symptoms occurs, rather than intermittently, only when you have symptoms.
  • This is particularly so for people with hay fever (seasonal allergic rhinitis). In the spring and summer months the pollen count is generally higher and you may be in contact with the allergen on a regular basis. Taking the medication regularly will help keep your symptoms under control.

Possible side effects?

Most people who take antihistamines do not have any serious side-effects. If side-effects do occur, they are usually minor.

 Minor side effects (these usually go way with time)
  • drowsiness
  • dry mouth, nose or throat
  • dizziness
  • headache
More serious side effects (tell your health care provider) 
  • fast or rapid heartbeat
  • unusual weakness
  • stomach pain
  • difficulty passing urine
  • problems with your eye sight (blurred vision) 
  • stomach pain and discomfort

When should I avoid antihistamines?

Talk to your doctor before you begin taking antihistamines, as they could interact with your other medications or might not be suitable if you have other health conditions. Remember that cough or cold remedies often contain antihistamines, so if you are taking these as well you need to work out your total dosage to make sure you don’t take more than the total recommended amount for any one period.

Some people use antihistamines such as Phenergan and Vallergan as sedatives for their children, for example when they are travelling on a plane, but this can backfire as some children respond by becoming significantly more active (not a nice thing at 30,000 feet). Always check with your GP before giving your children medications.

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Source: Health Navigator