Antihistamines for allergy

Sedating antihistamines can make you feel quite drowsy or sleepy. They are used for a short time, when the effect of drowsiness is helpful to the condition being treated such as for eczema flares where itchiness can cause sleep disturbance.

Examples of sedating antihistamines:

• dexchlorpheniramine (Polaramine®)
• doxylamine (Dozile®)
• promethazine (Phenergan®, Allersoothe®).

Take care when using sedating antihistamines

• Sedating antihistamines may affect your concentration and performance of some tasks that require you to be alert, eg, driving, and operating machinery. Take care until you know how these medicines affect you.
• Avoid alcohol while you are taking sedating antihistamines; alcohol can make the drowsiness worse.
• It's recommended that sedating antihistamines should not be used to treat allergic reactions. If used to treat a mild or moderate allergic reaction such as hives, it is hard to know whether the person is drowsy because of the sedating antihistamine or because of a drop in blood pressure due to a severe allergic reaction (anaphylaxis). Non-sedating antihistamines are preferred. 

Non-sedating antihistamines are less likely to cause drowsiness. Although drowsiness is unlikely to happen with these medicines, 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.

Less sedating, or non-sedating antihistamines are recommended to manage signs and symptoms of mild to moderate allergic reactions, such as hives, welts or swelling of the face, lips, eyes, tingling of the mouth.

Examples of non-sedating antihistamines:

• bilastine (Labixten®)
• cetirizine (Zista®, Histaclear®, Zyrtec®, Razene®, Zetop®)
• desloratadine (Aerius®)
• fexofenadine (Fexaclear®, Telfast®, Fexofast®)
• levocetirizine (Levoclear®, Levrix®)
• loratadine (Loraclear®, Claratyne®, Lora-tabs®, Lorafix®).

How quickly antihistamines work can vary depending on the medicine and the person. Antihistamine tablets usually start to work within 30 minutes and are likely to be most effective within 1 to 2 hours.

• For allergies such as hay fever, antihistamines are more effective when taken regularly as a prevention, before symptoms occur, rather than only when you have symptoms.
• 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.

Here are some things to know before you take antihistamines. Other things may be important as well, so ask your healthcare provider what you should know about.

If you have some health conditions

Sedating antihistamines, in particular, may not be suitable for some people. If you have any health problems, let your healthcare provider know before you start taking any antihistamines. Here are a few examples:

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

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

If you are driving, cycling or operating machinery

Antihistamines can affect your concentration (how you focus or pay attention). They may slow your reaction times and cause sleepiness. This is more likely with sedating antihistamines but can some times happen with non-sedating antihistamines. Don't drive, cycle or operate machinery until you know how antihistamines affects you. See driving and medicines.

Increased risk of falls

Antihistamines, especially sedating antihistamines, can increase the risk of falls, especially in older adults. The risk is greater if you are taking multiple medications that affect the central nervous system such as pain medicines or in people with balance problems or chronic conditions like Parkinson’s or neuropathy. Non-sedating antihistamines are less likely to cause drowsiness or dizziness. Always start with the lowest effective dose and avoid drinking alcohol or taking other sedating medicines. Read more about medicines and falls risk. 

Like all medicines, antihistamines can cause side effects, although not everyone gets them. If you're concerned about any symptoms you think might be related to your medicine, talk to your healthcare provider. The following information offers some guidance but doesn't include all possible side effects.

Common side effects

Tell your doctor if you are concerned.

• Drowsiness, dizziness: Be careful when driving, cycling or using tools until you know how this medicine affects you. These effects put you at risk of falls, and injuries especially if you are elderly. Don't drink alcohol.
• Dry mouth, nose or throat
• Headache

Tell your healthcare provider immediately or phone Healthline free on 0800 611 116 if these occur

• Fast or rapid heartbeat, trouble breathing
• Unusual weakness
• Difficulty passing urine
• Problems with your eye sight (blurred vision)

Read more about medicines and side effects and reporting a reaction you think might be a side effect.