Content Supplied by NHS Choices

Antihistamines

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

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

These include:

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.

Content Supplied by NHS Choices

Antihistamines

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

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:

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.

Content Supplied by NHS Choices

Antihistamines

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

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.

Content Supplied by NHS Choices

Antihistamines

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

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:

Share this page