Antihistamines relieve the symptoms of allergies such as:
Sometimes your GP will recommend antihistamines:
- to prevent motion sickness
- as a short-term treatment for sleep problems in adults
You can buy most antihistamines from a pharmacy without a prescription. These include chlorpheniramine, cetirizine and fexofenadine.
But you will need a prescription for some types of antihistamine.
Antihistamines come as tablets, capsules, liquids, syrups, creams, lotions, gels, eye drops and nasal sprays.
Types of antihistamines
There are many types of antihistamine.
But they can be divided into 2 main groups - antihistamines that:
Make you feel sleepy
These include promethazine (Phenergan) or chlorphenamine (Piriton). Antihistamines that make you feel sleepy can help if your symptoms cause sleep problems.
Make you feel less sleepy
These are sometimes called non-drowsy antihistamines. They include cetirizine (Anti-Hist, Cetrine, Zirtek and Zirtene), fexofenadine (Telfast) and loratadine (Clarityn and Lorat).
Do not drive or use machinery after taking any type of antihistamine.
All types of antihistamine work well. But some people find certain types work better than others. You may need to try a few types to find an antihistamine that works for you. Ask a pharmacist for advice.
Check if you can take antihistamines
Most people can safely take antihistamines. But not all antihistamines are suitable for everyone.
Read the leaflet that comes with your medicine to check it's safe for you or your child to take.
Non-urgent advice: Ask a pharmacist or GP for advice if
you or the person you are buying an antihistamine for:
- is pregnant
- is trying to get pregnant
- is breastfeeding
- takes other medicines
- has an underlying health condition, such as heart disease, liver disease, kidney disease or epilepsy
- is age 11 or under
Children and antihistamines
Some antihistamines are suitable for children. These include promethazine (Phenergan) and cetirizine (Anti-Hist, Cetrine, Zirtek and Zirtene). You can give these antihistamines to children age 2 and older.
Antihistamine liquids may be easier for children to take than tablets. The liquid medicine comes with a plastic syringe or spoon to give the right dose. If you do not have a syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not measure out the right amount.
Always read the leaflet that comes with your medicine to see if it's safe and check the correct dose when giving it to your child.
Non-urgent advice: Find your patient information leaflet
Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.
How and when to take antihistamines
Follow your GP or pharmacist's advice on how and when to take your antihistamine. The leaflet that comes with your medicine also has advice on how to take them.
How much you should take can depend on your age and weight. Some types can make you feel sleepy. Take this type of antihistamine just before you go to bed.
Talk to your GP or pharmacist if you've been given eye drops or a nasal spray. They can show you how to use it correctly.
You can use some types of antihistamine for a long time. But some are only recommended for a few days.
Eating and drinking
Do not drink alcohol while you are taking an antihistamine. Some antihistamines make you feel sleepy. Alcohol can increase the chances of this happening.
You can have most other foods and drinks. But check the leaflet that comes with your medicine to make sure.
Check the leaflet or ask your GP or pharmacist if you should take antihistamine tablets or liquids with food or water.
Taking antihistamines with other medicines
Talk to a pharmacist or GP before taking antihistamines if you take other medicines.
There may be a risk the medicines do not mix. This could stop 1 of them from working properly. It can also increase the risk of side effects.
This can happen with some types of:
- antidepressants
- stomach ulcer or indigestion medicines
- cough and cold remedies that also contain an antihistamine
Side effects of antihistamines
Like all medicines, antihistamines can cause side effects.
There are things you can do to manage common side effects:
Sleepiness
Do not drive, cycle or use tools or machinery if you're feeling this way. Do not drink alcohol as this will make you feel more tired.
Dry mouth
Try chewing sugar-free gum or sucking sugar-free sweets.
Blurred vision
Do not drive, cycle or use tools or machines until you can see properly again.
Difficulty peeing
Relax when you try to pee. Do not try to force the flow of pee. If you cannot pee, try again a short time later. Talk to your doctor urgently if you cannot pee at all.
Headaches
Rest and drink plenty of fluids. Do not drink alcohol. Ask your pharmacist to recommend a painkiller. Talk to your GP if the headaches last longer than a week or are severe.
Feeling sick (nausea)
Check the leaflet that comes with your medicine for a full list of possible side effects and advice about when to get medical help.
Serious side effects
Serious side effects from an antihistamine are rare. But you will need urgent medical help if you have serious side effects.
Urgent advice: Contact your GP immediately if you have:
- bruising or bleeding that is more than normal
- yellowing of your skin or whites of your eyes - these can be signs of liver problems
- a fast or irregular heart rate
- muscle stiffness or shaking, or unusual face or tongue movements
- numbness or pins and needles
- confusion or restlessness
- tiredness that lasts for a long time
- overactive behaviour, particularly in children
- difficulty peeing
Stop taking your antihistamine straight away.
Serious allergic reaction
A severe allergic reaction (anaphylaxis) to antihistamines is rare. But you will need to go to an emergency department (ED) if you get symptoms.
Emergency action required: Call 112 or 999 or go to an ED if:
- you get a rash that's swollen, raised, itchy, blistered or peeling
- you find it hard to swallow
- you have breathing problems such as wheezing or gasping for air
- your lips, face, throat or tongue start swelling
- your skin, tongue or lips turn blue, grey or pale
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do
Stop taking your antihistamine straight away.
Non-urgent advice: Find your patient information leaflet
Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.
Report side effects
You can report any suspected side effects to the Health Products Regulatory Authority (HPRA): report an issue - hpra.ie
How antihistamines work
Antihistamines block the effects of a chemical in your body called histamine.
Your body releases histamine when it detects something harmful, such as an infection. Histamine causes blood vessels to expand and the skin to swell. This helps protect the body.
If you have allergies, you may produce histamine by mistake. This is because your body mistakes something harmless - such as pollen or animal hair - for a threat.
The histamine causes an allergic reaction with itchy, watering eyes, a running or blocked nose, sneezing and rashes.
Antihistamines help stop this happening if you take them before you are near the allergen (thing that causes an allergic reaction). Or they can stop symptoms getting too bad if you take them afterwards.
Fact check
This content was fact checked by a pharmacist, a GP, the National Medication Safety Programme (Safermeds) and the Health Products Regulatory Authority (HPRA).
