Introduction 
 


Antihistamines - Side effects


 

Antihistamines are a type of medicine often used to treat a number of allergic health conditions.

Although antihistamines can't cure these conditions, they often provide relief from symptoms. For example, antihistamines may be used to treat:



hay fever



allergic rhinitis – inflammation of the nose caused by an allergic reaction to substances such as dust mites



allergic skin conditions, such as eczema or urticaria (hives)



allergic conjunctivitis – inflammation of the eyes



allergic reactions caused by insect bites or stings



mild or moderate allergic reactions caused by food allergies – more severe allergic reactions (anaphylaxis) usually require emergency treatment with adrenaline



Antihistamines also have a number of other uses, such as treatingstomach ulcers, insomnia (problems falling asleep) and motion sickness.

Antihistamines are available as



tablet or capsules (oral antihistamines)



creams, lotions and gels (topical antihistamines)



a nasal spray



Many antihistamines are available over the counter at a pharmacy, although some require a prescription.

How antihistamines work

Antihistamines work by altering the way cells are affected by a substance called histamine. Histamine is a chemical the immune system uses to help protect the body's cells against infection.

Usually histamine is a useful substance, but if you're having an allergic reaction it's sometimes necessary to block its effects. Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, for a threat.

Types of antihistamine and their effects

Antihistamine medicines are classified in three groups. These are:



first-generation antihistamines – which cause drowsiness in most people and include diphenhydramine and chlorphenamine 



second- or third-generation antihistamines – which are less likely to cause drowsiness and include loratadine and cetirizine



Second- or third-generation antihistamines are usually recommended. Don't underestimate the levels of drowsiness caused by first-generation antihistamines – their effects can continue into the next day if you take them at night.

An exception to this is sometimes made if the drowsiness caused by first-generation antihistamines can be beneficial, for example in cases where itchy skin may be causing sleep problems.

Safety

Even though most antihistamines are available without a prescription, you shouldn't assume they're safe for everyone to take.

Antihistamines may have dangerous and unpredictable effects if taken by people with certain conditions or if combined with certain other substances, such as alcohol or certain antidepressants.

It's also important to only take antihistamines as directed. Overdoses are possible and overuse can lead to you becoming reliant on the sedating effects.

Before taking antihistamines, always read the patient information leaflet that comes with the medicine to check the safety information.

Hay fever advice








 




Hay fever is an allergy to pollen that affects around one in four people. An expert explains how it's diagnosed, the symptoms and treatment.


Media last reviewed: 19/03/2013

Next review due: 19/03/2015

How antihistamines work 

 

Antihistamines work by stopping histamine affecting your body's cells in the usual way. They target special molecules called receptors, which are found in your cells.

Histamine

Histamine is a chemical the immune system uses to help protect the body's cells against infection. The immune system is the body's natural defence against illness and infection.

If the immune system detects a harmful foreign object, such as bacteria or a virus, it will release histamine into nearby cells. The histamine causes small blood vessels to expand and the surrounding skin to swell. This is known as inflammation.

Histamine is usually a useful substance, but if you're having an allergic reaction it's sometimes necessary to block its effects. Allergic reactions occur when your immune system mistakes a harmless substance, such as pollen, for a threat.

Receptors

Receptors are molecules found in the cell walls. They react when they come into contact with certain substances.

Antihistamines work by blocking the receptors in each cell, so histamine can't activate the receptors and affect the cell.

Histamine receptors

Histamine receptors cause inflammation and stimulate the production of stomach acid. They're also thought to help stimulate chemicals that transmit information around the brain and may help regulate the immune system.

The majority of antihistamines are designed to block the receptors causing inflammation. Antistamines used to treat stomach ulcers are designed to block the receptors producing stomach acid.

 

Research is underway to produce an antihistamine which can be used to treat mental health conditions and neurological conditions, such asAlzheimer’s disease and autoimmune conditions, such as rheumatoid arthritis.

Who can use antihistamines 

 

Most people are able to take antihistamines. However, antihistamines are not recommended in certain circumstances.

These are explained below.

Health conditions

Certain conditions can be made worse by taking antihistamines, or they can cause the antihistamines to react unpredictably. These include:



asthma



diabetes (type 1 diabetes and type 2 diabetes)



high blood pressure



epilepsy



heart disease



liver disease



kidney disease



an overactive thyroid gland



Before taking antihistamines, seek advice from your GP or pharmacist if you have a health condition.

Pregnancy

As a general rule, avoid taking any medication during pregnancy unless there's a clear clinical need. Always check with your GP, pharmacist or midwife first.

If you're pregnant and feel you need antihistamines, nasal sprays, nose drops or eye drops will at first be recommended. If these don't work, an oral antihistamine, usually loratadine or cetirizine is likely to be recommended.

Chlorphenamine is also considered safe to use during pregnancy, but should be avoided close to labour and childbirth as it can cause problems in the baby, such as irritability or tremor (shaking).

Breastfeeding

It may be possible for you to take some hay fever medicines while you're breastfeeding without risk to your baby. 

However, you should always get advice from your pharmacist, GP or health visitor first.

Children

Some antihistamines, such as alimemazine and promethazine, aren't suitable for children under two years old. You should seek advice from your GP if your child is under two years old and you think they require treatment with antihistamines.

Some antihistamines are not recommended for children with certain conditions. For example, the National Institute for Health and Care Excellence (NICE) recommends oral antihistamines aren't used routinely to treat children with atopic eczema.

Before giving your child any form of medication, read the patient information leaflet for advice about whether the medication is suitable for them.

Interactions with other medicines  

 

The effect of antihistamines can sometimes be altered when they're combined with other substances.

This is known as "interaction" and it's important to try to avoid this whenever possible, as the effects can be unpredictable and potentially dangerous.

First-generation antihistamines

Avoid drinking alcohol when taking first-generation antihistamines because this will increase feelings of drowsiness.

This is the same for other types of medication known to have a sedating effect, such as:



sleeping tablets



benzodiazepines, often used to treat anxiety disorders



tricyclic antidepressants, used to treat a range of mental health conditions, such as depression and obsessive compulsive disorder (OCD), as well as a number of chronic pain conditions



Speak to your GP or pharmacist before taking a first-generation antihistamine if you're taking any of the above medicines.

You shouldn't take a first-generation antihistamine if you're also taking a type of antidepressant known as a monoamine oxidase inhibitor (MAOI). This is because the combination of the two substances can have unpredictable effects.

Second- and third-generation antihistamines

Most second- and third-generation antihistamines don't interact with other medicines. However, the exceptions to this are:



rupatadine – this can cause unpredictable effects if taken with some types of antibiotics or grapefruit juice



mizolastine– this can cause unpredictable effects if taken withnifedipine (used to treat high blood pressure), cimetidine (used to treat heartburn) and ciclosporin (often used to treat people who've had an organ transplant)



Cough and cold medicines

Many cough and cold medicines available over the counter at pharmacies contain a mixture of different medications, such as paracetamol, decongestants and antihistamines.

Don't take cough and cold medicines if you've recently taken other antihistamine medication because there's a risk of taking an excess dose.

These types of cough and cold medicines aren't recommended for children under six years old because the risks of treatment are thought to outweigh any benefits.

Side effects of antihistamines 

 

Like all medicines, antihistamines can have side effects. Generally, these are more significant with first-generation antihistamines.

For a full list of specific side effects of your medicine, see the information leaflet that comes with your medication.

Most information leaflets can also be found online on the Medicines and Healthcare products Regulatory Agency (MHRA) or electronic Medicines Compendium (eMC) websites.

First-generation antihistamines

Common side effects of first-generation antihistamines include:



drowsiness



impaired thinking



dry mouth



dizziness



constipation



blurred vision



an inability to fully empty the bladder (urinary retention)



It's important not to underestimate the effects of antihistamine-related drowsiness. Some first-generation antihistamines can impair co-ordination, reaction times and judgement in the same way that alcohol consumption can. Therefore you shouldn't drive or use power tools or heavy machinery after taking a first-generation antihistamine.

Less common side effects of first-generation antihistamines include:



insomnia (difficulty sleeping)



nightmares



hallucinations (seeing or hearing things that aren't real)



itchy skin



Rare side effects of first-generation antihistamines include:



rapid heartbeat



chest tightness



Contact your GP if you experience these rare side effects.

Side effects in children

 

Children have a greater risk of side effects from first-generation antihistamines, particularly drowsiness and impaired thinking.

Second- or third-generation antihistamines

Second- or third-generation antihistamines are less likely to cause drowsiness. If you find yourself feeling drowsy, don't drive, drink alcohol, or use tools or machines.

Other side effects of second- or third-generation antihistamines include:



headache



dry mouth



dry nose



feeling sick



These side effects don't usually last long and should pass quickly.

Rarer side effects include:



rapid heartbeat



chest tightness



However, second- and third-generation antihistamines have been found to have less risk of heart problems than first-generation antihistamines.

Contact your GP if you experience these rare side effects.

H2 receptor antagonists

Antihistamines used to treat stomach ulcers are known as H2 receptor antagonists. Side effects of this type of antihistamine are uncommon but may include:



diarrhoea



headache



dizziness



skin rashes



tiredness



 

Yellow Card Scheme

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine you're taking.

It's run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

See the Yellow Card Scheme website for more information.