Antihistamines

How do antihistamines work?

Allergic symptoms occur when your body wrongly recognises a food or something in your environment (such as pollen spores) 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 and non-sedating antihistamines.

Sedating antihistamines

Sedating antihistamines 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. Take care until you know how these medicines affect you.
• Limit or avoid alcohol while you are taking sedating antihistamines – alcohol can make the drowsiness worse.

Use in children

Sedating antihistamines are often found in cough and cold medicines. These medicines are not recommended for children under 6 years of age. Read more about cough and cold medicines in children. 

Sedating antihistamines are not recommended in children under 2 years of age for other conditions.

Non-sedating antihistamines

Non-sedating antihistamines are less likely to cause drowsiness. Although drowsiness is unlikely to happen, it can still occur in some people and may affect the performance of skilled tasks such as driving. Take care until you know how these medicines affect you.

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 occur, rather than 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 often. Taking the medication regularly will help keep your symptoms under control.

Precautions – before taking antihistamines

Sedating antihistamine, in particular, may not be suitable for some people. If you have any of the following problems, let your doctor or pharmacist know before you start taking any antihistamines:

• glaucoma
• problems with your liver or kidneys
• epilepsy
• acute porphyria (a rare metabolic disorder)
• men with prostate enlargement (benign prostatic hyperplasia).

If you are pregnant or breast-feeding check with your doctor before taking antihistamines. 

Possible side effects

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

References

1. Anhistamines New Zealand Formulary
2. Changes Regarding the Use of Sedating Antihistamines Medsafe Sept 2018