Antihistamines

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

These include:

For more information about the conditions that this type of medicine can help treat, see Antihistamines - what it is used for.

Antihistamines work by blocking the effects of a protein called histamine (see below). They're available in tablet or capsule form (oral antihistamines), creams, lotions and gels (topical antihistamines) and as a nasal spray.

How antihistamines work

Histamine is a protein 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.

The expansion of the blood vessels allows an increased number of infection-fighting white blood cells to be sent to the site of the infection. The swelling of the surrounding skin also makes it harder for an infection to spread to other parts of the body.

Histamine is usually a useful protein, 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, as a threat.

The release of histamine causes the process of inflammation to begin and leads to nearby tissue becoming red and swollen. It can also affect the nerves in the skin, making the skin feel itchy.

Types of antihistamine

There are a number of antihistamine medicines, which are classified in two groups. These are:

  • first-generation antihistamines, which cause symptoms of drowsiness in most people; they include diphenhydramine and chlorphenamine 
  • second-generation antihistamines, which do not usually causes symptoms of drowsiness and include loratadine and cetirizine

Second-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.

Research has found that adults who regularly take first-generation antihistamines are more likely to be involved in serious accidents. Similarly, children who regularly take first-generation antihistamines perform less well at school than would usually be expected.

An exception to these recommendations is sometimes made if the drowsiness caused by first-generation antihistamines can be of some use. For example, if you're having problems sleeping because you have itchy skin.

Where do I get them?

Many antihistamines are available direct from your pharmacist without prescription.

Safety

Even though antihistamines are a non-prescription medication, you shouldn't assume that they're safe for everyone to take.

It's also important to only take antihistamines as directed. Overdoses are possible and overuse can lead to addiction.

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

For more information see Antihistamines - who can use it.

Allergen
An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.
Allergic
An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Drowsiness
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Fever
A fever is when you have a high body temperature (over 38°C or 100.4°F).
Heart
The heart is a muscular organ that pumps blood around the body.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses. 
Sneezing
Sneezing is an involuntary expulsion of air and bacteria from the nose and mouth.
Vomiting is when you bring up the contents of your stomach through your mouth.
Stomach
The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.
Swelling
Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Ulcers
An ulcer is a sore break in the skin, or on the inside lining of the body.

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

Although antihistamines can't cure these types of conditions because they don't affect the underlying cause, 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 and hives
  • allergic conjunctivitis - inflammation of the eyes
  • allergic reactions that are caused by insect bites or insect stings
  • mild allergic reactions that are caused by food allergies; more serious 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) and insomnia (problems falling asleep).

Stomach ulcers

A type of antihistamine, known as a H2-receptor antagonist, is sometimes used to treat stomach ulcers. This is because histamine can also stimulate the production of stomach acid.

H2-receptor antagonists can be used to block the 'acid-stimulating' effect of histamine, which helps to reduce the acid level in the stomach and digestive system.

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 for more information.

Adrenaline
Adrenaline is a hormone produced at times of stress that affects heart rate, blood circulation and other functions of the body.
Allergic
An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.
Anaphylactic shock
Anaphylactic shock is a severe and sometimes life-threatening allergic reaction, causing swelling of body tissues and a drop in blood pressure.
Anxiety
Anxiety is an unpleasant feeling when you feel worried, uneasy or distressed about something that may or may not be about to happen.
Chronic
Chronic usually means a condition that continues for a long time or keeps coming back.
Congestion
Congestion is an excess of fluid in part of the body, often causing a blockage.
Decongestant
Decongestant medicine relieves congestion by reducing the swelling of the lining the nose and sinuses and drying up the mucous.
Drowsiness
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Fever
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).
Morning sickness
Morning sickness refers to the nausea and vomiting experienced early in a pregnancy, not necessarily in the morning.
Sickness
Vomiting is when you bring up the contents of your stomach through your mouth.
Sneezing
Sneezing is an involuntary expulsion of air and bacteria from the nose and mouth.
Stomach
The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.
Swelling
Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Ulcers
An ulcer is a sore break in the skin, or on the inside lining of the body.

Antihistamines don't prevent the body from producing histamine, but they stop 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.

Receptors

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

To understand how receptors work, it's useful to imagine a cell as a machine and the receptors as switches that are capable of changing how the machine works. However, the switches can only be used when the right key (the histamine protein) is used to activate the switch.

Antihistamines work by blocking the receptor sites in each cell in the same way that superglue can block a lock, so that the histamine 'key' 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
  • H2
  • H3
  • H4

The H1 receptor is the receptor that causes inflammation. Therefore, the majority of antihistamines are designed to block the H1 receptor.

The H2 receptor is the receptor that helps to stimulate the production of stomach acids. Therefore, antihistamines that are used to treat stomach ulcers are designed to block the H2 receptor.

The H3 receptor seems to play an important role in stimulating the production of neurotransmitters, which are 'messenger chemicals' used by brain cells to transmit information around the brain. Neurotransmitters can have a powerful effect on your mood and emotional state.

The H4 receptor has only recently been discovered and not much is known about its role, although it's thought to be involved with the regulation of the immune system.

At present, there are no commercially available antihistamines that can block the H3 or H4 receptors. However, current 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.

Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Brain
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
Drowsiness
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Fever
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).
Sneezing
Sneezing is an involuntary expulsion of air and bacteria from the nose and mouth.
Stomach
The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.
Tissues
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  
Ulcers
An ulcer is a sore break in the skin, or on the inside lining of the body.
Vomit
Vomiting is when you bring up the contents of your stomach through your mouth.

Most people are able to take antihistamines. However, antihistamines are not recommended in certain circumstances, which 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. Before taking antihistamines, seek advice from your GP or pharmacist if you have:

  • asthma
  • diabetes (type 1 diabetesand type 2 diabetes)
  • high blood pressure
  • epilepsy
  • glaucoma - an eye condition that's associated with a build-up of fluid inside the eyes
  • an enlarged prostate gland
  • heart disease
  • liver disease
  • kidney disease
  • a bladder obstruction, such as a bladder stone
  • an overactive thyroid gland
  • a blockage in your stomach or intestines

Pregnancy and breastfeeding

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, or you're advised to take them by your GP, loratadine or chlorphenamine are the antihistamines that considered the safest to use.
However, chlorphenamine is a first-generation antihistamine, so it may make you feel drowsy.

If you can't take loratadine or chlorphenamine, your GP may recommend another oral antihistamine called cetirizine. Cetirizine is also considered safe to use during pregnancy.

Antihistamine eye drops and nasal sprays should be used with caution during pregnancy. Some people are allergic to antihistamine eye drops. Seek advice from your GP before using antihistamine eye drops or nasal sprays.

Most antihistamines will pass into breast milk. While this isn't thought to be particularly harmful, it's recommended as a precaution that you don't breastfeed while taking antihistamines.

Children

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

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

Antibiotics
Antibiotics are medicines that can be used to treat infections caused by micro-organisms, usually bacteria or fungi. For example, amoxicillin, streptomycin and erythromycin.
Antifungal
Antifungal medicine is used to treat fungal infections. For example, clotrimazole, ketoconazole.
Dose
Dose is a measured quantity of a medicine to be taken at any one time, such as a specified amount of medication.
Drowsiness
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Heart
The heart is a muscular organ that pumps blood around the body.
Kidney
Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body as urine.
Liver
The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.

First-generation antihistamines

Avoid drinking alcohol when taking first-generation antihistamines because this will make the feeling of drowsiness worse.

The same is true for other types of medication that are known to have a sedating effect, such as:

  • sleeping tablets
  • benzodiazepines, which are often used to treat anxiety disorders
  • tricyclic antidepressants, which are 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 (which is often used to treat people who've had an organ transplant)

Cough and cold medicines

Many cough and cold medicines that are 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 of antihistamine.

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

First-generation antihistamines

Common side effects of first-generation antihistamines include:

  • drowsiness
  • impaired thinking
  • dry mouth
  • dizziness

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 judgment 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.

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:

Antihistamines
Antihistamine medicine counteracts the action of histamine (a chemical released during an allergic reaction). For example, loratadine, hydroxyzine.
Drowsiness
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Fever
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).
Heart
The heart is a muscular organ that pumps blood around the body.

 

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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