Antihistamines how do they work

Whereas, in the stomach, histamine made by cells that line the stomach helps to produce acid for food digestion. Your immune system cells monitor your blood and mucosae for anything for example, germs such as bacteria or viruses that is not made by your body. Mucosae are membranes lining body cavities such as your mouth, nose and digestive tract. If your skin is damaged or your immune system detects a foreign substance, histamine is released from mast cells. The histamine binds to special sites receptors on other cells, called H1 receptors.

This sets off a chain reaction which causes blood vessels in the area to become slightly leaky. Specialised cells and chemicals, which defend your body, can now get access to the area. While this is a helpful response, it also causes redness, swelling and itching. Allergic reactions such as hay fever seasonal allergic rhinitis are caused by an oversensitivity or over-reaction of the immune system to a particular allergen.

An allergen is a substance that is foreign to the body and which can cause an allergic reaction in certain people. For example, pollen, dander, mould, some germs. In most people, the immune reaction to these foreign substances is normal and appropriate. But in allergic people, it is excessive. For example, in people with hay fever, contact with pollen in the nose, throat and eyes triggers the mast cells there to release much more histamine than normal.

This excessive release of histamine produces the associated symptoms of itching, swelling, runny eyes, etc. Antihistamines work by physically blocking the H1 receptors, stopping histamine from reaching its target. This decreases your body's reaction to allergens and therefore helps to reduce the troublesome symptoms associated with allergy.

Antihistamines are also used in the treatment of feeling sick nausea and being sick vomiting. However, the exact way that they ease these symptoms is not fully understood.

The brain has several key areas which control vomiting. It is thought that antihistamines block H1 receptors in the area of the brain which creates nausea in response to chemicals in the body.

Some antihistamines may also have what is known as an antimuscarinic effect. This means that the medicine can also block another type of receptor found on the surface of certain cells. If these receptors are affected, you may experience some of the side-effects associated with antihistamines.

For example, dry mouth, blurred vision and retention of urine. These effects are mainly caused by the older first-generation antihistamines which are described below. Note : antihistamines should not be confused with H2 blockers which reduce the production of stomach acid.

While both types of medicine block the actions of histamine, they work on different receptors in different systems of the body. Some of these medicines are available to buy over the counter from your pharmacist.

Others are only available on prescription. These medicines come in a variety of forms, as mentioned above. Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often. Read the leaflet that comes with your particular brand for further information.

All antihistamines work pretty well to reduce symptoms of allergy. Your doctor or pharmacist may advise or prescribe a particular antihistamine depending on the cause of your allergy and on whether you require a sedating or non-sedating medicine.

For other conditions, specific antihistamines may be used. For example, cyclizine and promethazine teoclate are used for feeling sick nausea and being sick vomiting , not for hay fever. Chlorphenamine is the antihistamine most used in an emergency situation such as anaphylaxis, and may be given by injection in this situation.

An antihistamine tablet typically starts to work within 30 minutes after being taken. The peak of effectiveness is typically within hours after being taken. This can vary depending on the reason for treating you.

If you have hay fever you may take the medicine throughout the pollen season. Most people who take antihistamines do not have any serious side-effects. If side-effects do occur, they are usually minor. The most common are:. For a full list of all the side-effects and possible interactions associated with your medicine, consult the leaflet that comes with your medication.

If you think you have had a side-effect to one of your medicines you can report this on the Yellow Card Scheme. However, terfenadine, which is one of the antihistamines associated with liver damage, is no longer available for clinical use. It is safe for pregnant women to take most first- and second-generation antihistamines at low doses.

However, taking high doses of promethazine, which is a first-generation sedating antihistamine, may cause adverse side effects during pregnancy. Pregnant women should speak with their doctor before taking antihistamines. Most healthcare providers recommend using second- or third-generation antihistamines to treat mild to moderate allergy symptoms, including congestion, watery eyes, and itchy skin.

People can still buy first-generation antihistamines. However, these forms can cause drowsiness and sedation. People can choose between a wide range of antihistamines in drugs stores and online.

Parents and caregivers may want to consult a health care professional before giving an antihistamine to a child, especially if the child is 12 years old or younger. Zyrtec and Claritin are two popular antihistamines that can reduce allergy symptoms such as sneezing, itching, and rashes. Zyrtec contains cetirizine…. Taking medications can mean other substances, such as alcohol, are more dangerous.

Mixing benadryl and alcohol has several distinct dangers to be…. Allergic reactions are numerous and varied, and the best treatments depend on the specific symptoms, such as rashes or sinus problems, and their…. In this article, learn about what eye drops are available for itchy eyes, when eye drops should be used, and when people should see a doctor. An allergy develops when the immune system becomes hypersensitive to a food or other substance. Learn how to identify and manage allergies here.

What are antihistamines? Medically reviewed by Alan Carter, Pharm. Overview Symptom relief Types Side effects and risks Summary We include products we think are useful for our readers. What are they, and how do they work? Share on Pinterest A person may take antihistamines to treat allergic rhinitis.

Or they can reduce the severity of symptoms if you take them afterwards. The leaflet that comes in the packet with your medicine will have detailed information about it, including how to take it and what side effects you might get.

If you no longer have the leaflet that came with your medicine, you can search for an online version of it using our medicines guide. You may also find information on individual antihistamines on these websites:. Page last reviewed: 28 February Next review due: 28 February Types of antihistamine There are many types of antihistamine.

They're usually divided into 2 main groups: antihistamines that make you feel sleepy — such as acrivastine , chlorphenamine Piriton , hydroxyzine and promethazine non-drowsy antihistamines that are less likely to make you feel sleepy — such as cetirizine , cinnarizine , diphenhydramine , fexofenadine and loratadine They also come in several different forms — including tablets, capsules, liquids, syrups, creams, lotions, gels, eyedrops and nasal sprays.

Which type is best? How to take antihistamines Take your medicine as advised by the pharmacist or doctor, or as described in the leaflet that comes with it. Before taking an antihistamine, you should know: how to take it — including whether it needs to be taken with water or food, or how to use it correctly if eyedrops or a nasal spray how much to take the dose — this can vary depending on things such as your age and weight when to take it — including how many times a day you can take it and when to take it some types should be taken before bedtime how long to take it for — some types can be used for a long time, but some are only recommended for a few days what to do if you miss a dose or take too much overdose The advice varies depending on the exact medicine you're taking.

Side effects of antihistamines Like all medicines, antihistamines can cause side effects. Side effects of antihistamines that make you drowsy can include: sleepiness drowsiness and reduced co-ordination, reaction speed and judgement — do not drive or use machinery after taking these antihistamines dry mouth blurred vision difficulty peeing Side effects of non-drowsy antihistamines can include: headache dry mouth feeling sick drowsiness — although this is less common than with older types of antihistamines Check the leaflet that comes with your medicine for a full list of possible side effects and advice about when to get medical help.

Taking antihistamines with other medicines, food or alcohol Speak to a pharmacist or GP before taking antihistamines if you're already taking other medicines. Examples of medicines that could cause problems if taken with antihistamines include some types of: antidepressants stomach ulcer or indigestion medicines cough and cold remedies that also contain an antihistamine Try not to drink alcohol while taking an antihistamine, particularly if it's a type that makes you drowsy, as it can increase the chances of it making you feel sleepy.

Who can take antihistamines Most people can safely take antihistamines. But speak to a pharmacist or GP for advice if you: are pregnant — read about taking hay fever medicines in pregnancy are breastfeeding — read about taking hay fever medicines while breastfeeding are looking for a medicine for a young child are taking other medicines have an underlying health condition, such as heart disease, liver disease, kidney disease or epilepsy Some antihistamines may not be suitable in these cases.