Antihistamines work by stopping histamine affecting your body’s cells in the usual way.

They do this by targeting special molecules called receptors, which are found in your cells.

Histamine

Histamine is a chemical that 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 and can lead to nearby tissue becoming red and swollen. It can also affect the nerves in the skin, making the skin feel itchy.

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 receptor sites in each cell, so that histamine can’t activate the receptors and affect the cell.

Histamine receptors

Four different histamine receptors are found in each cell. They are known as:

• H1 – the receptor that causes inflammation
• H2 – the receptor that helps stimulate the production of stomach acids
• H3 – the receptor that seems to help stimulate chemicals used to transmit information around the brain
• H4 – a receptor that is currently not well understood, although it may help regulate the immune system

The majority of antihistamines are designed to block the H1 receptor. Antihistamines used to treat stomach ulcers are designed to block the H2 receptor.

At present, there are no commercially available antihistamines that can block the H3 or H4 receptors. However, research is underway to produce such an antihistamine.

It’s thought that H3-blocking antihistamines could be useful in treating mental health conditions, such as depression, as well as neurological conditions, such as Alzheimer’s disease.

Recent research also suggests that H3-blocking antihistamines could be useful in helping to relieve neuropathic pain (pain caused by damage or irritation to the nerves).

It’s thought that an H4-blocking antihistamine may be useful in treating autoimmune conditions, such as rheumatoid arthritis, where the immune system attacks healthy tissue.

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

This is known as “interaction” and it is 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.

The same is true 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

Seek advice from your GP or pharmacist before taking a first-generation antihistamine if you’re taking any of the medications listed above.

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-generation antihistamines

Most second-generation antihistamines do not have important interactions when they’re taken with other medications. 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 with nifedipine (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 have 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.

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

These include:

• 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

Read more about what antihistamines are used for.

Antihistamines are available in tablet or capsule form (oral antihistamines), creams, lotions and gels (topical antihistamines) and as 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 that cells are affected by a substance called histamine. Histamine is a chemical that 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.

Read more about how antihistamines work.

Types of antihistamine and their effects

Antihistamine medicines are classified in three groups. These are:

• first-generation antihistamines, which cause symptoms of drowsiness in most people and include diphenhydramine and chlorphenamine 
• second-generation antihistamines, which do not usually cause symptoms of drowsiness and include loratadine and cetirizine
• third-generation antihistamines, which cause fewer serious side effects than second-generation antihistamines and include levocetirizine and fexofenadine

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

An exception to these recommendations is sometimes made if the drowsiness caused by first-generation antihistamines can be beneficial, such as cases where people have problems sleeping because of itchy skin.

Read more about the side effects of antihistamines.

Safety

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

For example, antihistamines may have dangerous and unpredictable effects if taken by people with certain conditions or if combined with certain other substances.

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.

Read more about who can use antihistamines and interactions 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 abilities such as co-ordination, reaction times and judgement in the same way that alcohol consumption can.

Therefore it’s very important that you do not 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.

Second-generation antihistamines

A few people will experience drowsiness after taking second-generation antihistamines. If you find yourself feeling drowsy, do not drive, drink alcohol or use tools or machines.

As well as drowsiness, other side effects of second-generation antihistamines include:

• headache
• dry mouth
• dry nose

These side effects are usually short-lasting and should pass quickly.

Rarer side effects include:

• rapid heartbeat
• chest tightness

Contact your GP if you have these rare side effects.

Third-generation antihistamines

Third-generation antihistamines have some similar side effects to second-generation antihistamines, including:

• drowsiness (although this is less common than with first-generation antihistamines)
• headache
• dry mouth
• feeling sick

However, third-generation antihistamines have been found not to have the same risk of heart problems.

H2 receptor antagonists

Antihistamines that are 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

Antihistamines are mainly used to help control symptoms of health conditions associated with allergic reactions.

Antihistamines can’t cure these types of conditions as they don’t affect the underlying cause, but they can often provide considerable symptom relief.

Conditions that can benefit from the use of antihistamines include:

• hay fever
• allergic rhinitis – inflammation of the nasal passages
• atopic eczema – a common allergic skin condition
• urticaria – also known as nettle rash or 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 treatment with adrenaline (a chemical that can reverse many of the processes associated with an allergic reaction)

Other uses

As well as being used to treat allergic conditions, antihistamines also have a number of other uses, including treating stomach ulcers (sores that develop on the lining of the stomach), insomnia (problems falling asleep) and motion sickness.

Stomach ulcers

Some types of antihistamine can be used to treat stomach ulcers by reducing the acid level in the stomach and digestive system. This is because histamine can stimulate the production of stomach acid.

Read more about treating stomach ulcers.

Insomnia

First-generation antihistamines may be of some benefit in the short-term treatment of insomnia, particularly if the symptoms of sleeplessness are caused by an underlying allergic condition, such as an allergic skin condition.

The long-term use of antihistamines to treat insomnia is not recommended because there are more effective treatments. In addition, there’s a risk that you could become addicted to the sedating effects of first-generation antihistamines.

Read more about treating insomnia.

Motion sickness

Antihistamines are sometimes also used to control the symptoms of nausea and vomiting associated with motion sickness.

Read more about treating motion sickness.

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

These are explained below.

Health conditions

A number of health 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 another health condition and you are unsure whether antihistamines are suitable for you.

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 before taking any medication.

If you’re pregnant and feel that you need antihistamines, your healthcare professional may first recommend nasal sprays, nose drops or eye drops.

If these do not work, they may recommend an oral antihistamine, usually loratadine or cetirizine.

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).

Read more information about taking hay fever medication during pregnancy.

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. They will take into account factors such as:

• how mild or severe your symptoms are – if your symptoms are mild, you may be able to manage without treatment
• how effective the medicine is
• how much of the medicine passes to your baby through your breast milk 

Read more information about taking hay fever medication while breastfeeding.

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 that oral antihistamines are not used routinely to treat children with atopic eczema.

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