Antihistamine

Antihistamines are a type of medication available over the counter, most commonly used to treat the symptoms associated with allergies.

Overview

The term ‘antihistamine’ can refer to a number of different medications, all of which are most commonly used to reduce and relieve the symptoms of minor allergic reactions. Drugs in this class include Azatadine, Brompheniramine, Cetirizine, Chlorpheniramine, Clemastine, Cyproheptadine, Desloratadine, Dexchlorpheniramine, Diphenhydramine, Doxylamine, Fexofenadine, Loratadine and Phenindamine. They are used by patients who have minor allergies which cause symptoms such as inflammation, rashes, hives, sneezing, coughing, runny nose, itching, dry throat and watering eyes. They are not suitable to treat severe allergies which may threaten the life of a patient.

These drugs work by inhibiting the histamine produced by the body and stopping it from being able to affect cell function. Histamine is the chemical which the body produces and releases when a harmful presence is detected, such as an infection or an unknown chemical substance. Histamine provokes certain reactions in the body as a defense against these unknown chemicals. It provokes the expansion of blood vessels, causing inflammation as a buffer to protect the body. This is an important defense mechanism against infection and foreign bodies, however, the problem occurs when people develop allergies. In these cases, the body perceives an ordinary substance, such as dust, pollen or animal hair, as a harmful threat to the body. It then provokes the release of histamine and this is what causes symptoms such as rashes, hives, sneezing and difficulty breathing.

Antihistamines work to prevent this reaction from happening. They stop the body from producing histamine as a response to allergy triggers and therefore prevent these symptoms from being experienced by the patient. Stopping the body from histamine is not dangerous, as the substances which most commonly trigger this type of allergic reaction are completely harmless, for example pollen or dust. The substances in themselves do not pose any risk to the allergy sufferer, it is the body’s reaction to these external triggers which causes the patient to suffer from unnecessary symptoms, discomfort and pain.

Antihistamines are simple drugs and are usually available to purchase very cheaply from pharmacist, chemists and even supermarkets. They are an incredibly accessible form of medication and have improved the day-to-day lives of millions of people around the world.

Antihistamines can either be taken as a precaution to prevent the reaction before coming into contact with the substance which the patient is allergic to, or after exposure to reduce and relieve the symptoms of an allergic reaction once the body has already responded. Some people choose to take this type of medication every day as a pre-emptive measure against external factors which are hard to avoid, such as pollen, dust and grass, all of which can trigger allergic reactions in some people. Other patients choose to take antihistamines only after their body has started to react to the allergy trigger. This route is often favored by patients whose allergies are triggered by less common substances such as horse hair or bee stings.

Antihistamines are most commonly taken in the form of tablets or capsules which are safe to take at any time of day and can be taken with or without food. In addition to oral routes, antihistamines are also available to take via parenteral or rectal methods and can be prescribed or purchased in any of the following forms:

  • Capsule
  • Solution
  • Tablet
  • Tablet, Chewable
  • Powder for Suspension
  • Liquid
  • Syrup
  • Tablet, Disintegrating
  • Capsule, Liquid Filled
  • Elixir
  • Suspension
  • Film
  • Capsule, Extended Release
  • Tablet, Extended Release
  • Suspension, Extended Release
  • Suppository

When antihistamines were first developed, they all came with the debilitating side effect of provoking severe drowsiness in almost all patients. Patients found that although taking the drugs seriously improved their reactions to allergies and alleviated most, if not all, day to day symptoms, the fatigue they experienced often stopped them from performing everyday tasks such as driving or concentrating and maintaining focus on a task. These antihistamines are now referred to as first-generation antihistamines and include the following drugs: brompheniramine, hydroxyzine and meclizine.

Since then, pharmaceutical companies have developed a second generation of antihistamines which do not cause drowsiness to such a debilitating extent. These antihistamines are the ones most commonly prescribed today and include the following drugs: cetirizine, loratadine and fexofenadine. If prescribed and taken in high doses, these newer antihistamines can still cause drowsiness, but to a far lesser extent than was caused by their first generation counterparts.

Antihistamines have improved the lives of millions of people with allergies around the world. They make it possible for people to carry on living their lives without having to worry about exposure to allergens on a daily basis. The ease of access and the low costs associated with this medication means that there are no barriers to people being able to benefit from the drugs.

Conditions treated

  • Mild allergic reactions
  • Hives
  • Rash
  • Congestion
  • Runny nose
  • Sneezing
  • Itchy throat
  • Hives
  • Skin rashes
  • Itching
  • Itchy or watery eyes
  • Motion sickness
  • Insomnia
  • Anxiety
  • Allergic rhinitis

Type of medicine

  • Antihistamine

Side Effects

Although they can be acquired through prescription, most antihistamines are available to purchase over the counter at pharmacies. This is because the drugs are not seen to be dangerous and therefore their distribution does not need to be closely controlled. In the vast majority of cases, modern antihistamines do not cause any side effects at all, but where there are side effects these are almost always minor and not harmful to the patient. However, this does not mean that antihistamines are entirely without side effects.

Antihistamines were developed in two waves. The first antihistamines to be created and prescribed are now referred to as first-generation antihistamines. These are far less common today as the side effects they induced were much more pronounced. They almost always caused severe drowsiness in the patient which could prevent people from going about their day-to-day lives as usual. The drowsiness meant that people could not drive or operate machinery if they were taking antihistamines, and in many cases, the disruption of the drowsiness could completely outweigh the benefits of the allergy relief.

In response to this limiting side effect, a second generation of antihistamines was developed and these are what are mostly used today. This second generation almost entirely removed the side effect of drowsiness from antihistamine medication and greatly reduced the severity of the drowsiness overall. Some patients still experience minor side effects, but these are far less severe than those experienced with first-generation antihistamines.

The most common side effects are all fairly minor, and include:

  • Dry mouth due to decreased saliva production
  • Dry eyes due to decreased tear production
  • Sleepiness and drowsiness
  • Dizziness and light-headedness
  • Nausea and vomiting
  • Difficulty passing urine
  • Blurred vision
  • Confusion
  • Lung secretions which are thicker than usual

Patients who regularly take antihistamines can become used to the side effects and they may start to dissipate over time. After a few weeks of taking antihistamines, the side effects may go away completely as the body becomes used to having the medication in its system. If patients do experience drowsiness when taking their antihistamine medication, it can be advisable to take the antihistamines just before bedtime so that the side effect of drowsiness cannot have an effect during the day. If antihistamines do make you drowsy, you shouldn’t drive or operate heavy machinery until this effect has completely worn off.

There are also a number of much less common side effects of antihistamines. These do not present themselves in many patients at all, but they have been attributed as side effects of antihistamines in some rare cases.

  • Blood Disorders
  • Increased Heart Rate
  • Auditory and Visual Hallucinations
  • Seizures
  • Difficulty Sleeping and/or nightmares
  • Diarrhea
  • Dry Nose
  • Dry Throat
  • Changes in mood, making the patient easily irritated, aggravated or angered
  • Sweating
  • Headaches
  • Hives
  • Uncontrollable shaking or shivers
  • Itching
  • Loss Of Appetite
  • Nervousness or anxiety
  • Problems With Eyesight
  • Rash
  • Tinnitus
  • Spinning sensation
  • Stomach Cramps
  • Constipation
  • Mental disturbances

Antihistamines can bring on these side effects in patients who have never suffered from them, but if patients are already experiencing any of the symptoms listed above due to an existing medical condition, the use of antihistamines can aggravate the severity of these symptoms. For this reason, patients with the following medical conditions should be especially wary of the side effects associated with antihistamines:

  • Epilepsy or seizures
  • Glaucoma
  • Prostate problems
  • Urinary retention
  • Thyroid disorders
  • High blood pressure
  • Diabetes

Generally speaking, second-generation antihistamines do not cause drowsiness in patients, unless taken or prescribed in doses which are higher than normal. The only exception to this is cetirizine, which can have quite cause quite significant sedation even in small doses. Fexofenadine is generally regarded to induce the least amount of sedation of all antihistamines. This, therefore, means that it is usually the drug recommended for drivers, pilots and those operating heavy machinery so that they can carry on their duties while still receiving treatment for allergies.

Dosage

Antihistamines can either be prescribed by a doctor or they can be recommended by a pharmacist and purchased over the counter. If you are purchasing your own antihistamines over the counter, be sure to read the instructions on dosing thoroughly and carefully, and if you have any questions about how much you should be taking, don’t hesitate to get in touch with a doctor or healthcare professional.

It is possible to take too many antihistamines, and this can cause harmful and potentially serious health conditions. It is vital that patients do not double up on doses, take tablets before their next dose is due or increase their dose without first seeking professional medical advice.

The information on doses listed below is based on the average for patients which fall into a certain category. As every patient is different, doctors may choose to change the dose to best reflect an individual’s particular condition. Do not change your dose without first seeking advice from a doctor or pharmacist.

The amount of medicine in a dose will vary depending on the strength of the active ingredients in the medicine. The regularity with which you take the medication and the length of time you take it for will all depend on the condition being treated and the individual circumstances of the patient.

Azatadine (tablets)

Adults should take between one and two milligrams of the drug as required, with a minimum of eight hours between doses.

Children over the age of 12 should take between 0.5 milligams to 1 milligram of the drug as required, with a minimum of 12 hours between doses.

Children aged between 4 and 12 years old should only take the drug if prescribed by a doctor. If a prescription of this drug is recommended, the doctor will provide full details of dosage.

This drug is not recommended for children and infants under the age of four.

Brompheniramine (capsules, tables and liquid)

Adults and children over 12 should take no more than four milligrams of the drug as required, with a minimum of four hours between doses.

Children between the ages of 6 and 12 should take no more than two milligrams of the drug as required, with a minimum of four hours between doses.

Children aged between four and six should take no more than 1 milligram with a minimum of four hours between doses.

This drug is not recommended for children and infants under the age of four.

Brompheniramine (injection)

Adults and children over the age of 12 should take no more than 10 milligrams of this drug, injected under the skin, into a muscle or into a vein with a minimum of eight hours between doses.

For children under the age of 12, the dose will vary depending on the body weight of the child in question.

This drug is not recommended for children and infants under the age of four.

Cetirizine (tablets and syrup)

Adults and children over the age of six should take between five and 10 milligrams of this drug once a day.

Children who are aged between four and six should take either up to five milligrams once a day, or a dose of 2.5 milligrams every 12 hours.

This drug is not recommended for children and infants under the age of four.

Chlorpheniramine (short-acting tablets or liquid)

Adults and children over the age of 12 should take no more than four milligrams of chlorpheniramine with a minimum of four hours between doses.

Children aged between six and 12 years old should take no more than two milligrams of the drug up to four times a day as required.

Children aged between four and six should only take this drug if advised by the doctor. Dosage instructions from the doctor should be followed carefully.

This drug is not recommended for children and infants under the age of four.

Chlorpheniramine (long-acting tablets or liquid)

For adults, a dose of between eight and 12 milligrams of the drug every eight to 12 hours is suggested.

For children over the age of 12, up to eight milligrams should be taken with a minimum of 12 hours between doses.

For children between the ages of four and 12, this drug should only be taken if prescribed by a doctor. The dose on the prescriptions should be followed carefully.

This drug is not recommended for children and infants under the age of four.

Chlorpheniramine (injection)

Adults should take between five and 40 milligrams of this drug, injected under the skin, into a muscle or into a vein with a minimum of eight hours between doses.

For children over the age of four, the dose will vary depending on the body weight of the child in question.

This drug is not recommended for children and infants under the age of four.

Clemastine (tablets and liquid)

Adults and children over the age of 12 should take a dose of 1.34 milligrams twice a day or a dose of 2.68 milligrams one to three times a day as required.

Children between the ages of six and 12 should take a dose of between 0.67 and 2.4 milligrams twice a day.

For children aged between four and six, this drug should only be taken if prescribed by a doctor. The dose on the prescriptions should be followed carefully.

This drug is not recommended for children and infants under the age of four.

Cyproheptadine (tablets or liquid)

For adults and children aged 14 years and above, patients should take up to four milligrams every eight hours. This dose may be increased by the doctor if required.

Children between the ages of six and 14 should take a dose of four milligrams of the drug every eight to 12 hours as required.

Children between 4 and 6 years of age can take up to 2 milligrams of the drug every eight to 12 hours as required.

This drug is not recommended for children and infants under four years of age.

Desloratadine (tablets)

For adults and children aged 12 years and above, patients should take up to five milligrams once a day.

Children between the ages of four and 12 should not take this drug unless specifically recommended by a doctor.

This drug is not recommended for children and infants under four years of age.

Dexchlorpheniramine (short-acting tablets or liquid)

For adults and children aged 12 years and above, patients should take up to two milligrams every four to six hours as required.

Children between the ages of five and 12 should take a dose of one milligram of the drug every four to six hours as required.

Children between four and six years of age can take up to 0.5 milligrams of the drug every four to six hours as required.

This drug is not recommended for children and infants under four years of age.

Dexchlorpheniramine (long-acting tablets or liquid)

Adult patients should take between four and six milligrams every eight to 12 hours as required.

Children should only take this drug if it is recommended by a doctor. In this case, the doctor will set the dose as necessary.

This drug is not recommended for children and infants under four years of age.

Diphenhydramine (capsules, tablets or liquid)

Adults and children over the age of 12 should take between 25 and 50 milligrams every four to six hours as required.

Children between the ages of six and 12 years old should take between 12.5 and 25 milligrams every four to six hours as needed.

Children between four and six years of age can take 6.35 to 12.5 milligrams of the drug every four to six hours as necessary.

This drug is not recommended for children and infants under four years of age.

Diphenhydramine (injection form)

Adults should take between 10 and 50 milligrams of this drug, injected into a muscle or into a vein.

For children over the age of four, the dose will vary depending on the body weight of the child in question. Injections can be administered up to four times a day.

This drug is not recommended for children and infants under the age of four.

Doxylamine (tablets)

Adults and children over the age of 12 can take up to 25 milligrams of doxylamine as required with a minimum of four hours between doses.

Children between the ages of six and 12 can take up to 12.5 milligrams as required with a minimum of four hours between doses.

Children between the ages of four and six should not take this drug unless it has been recommended and prescribed by a doctor.

This drug is not recommended to give to children under the age of four years old.

Fexofenadine (capsules)

Adults and children over the age of twelve can take up to 60 milligrams of Fexofenadine twice a day. In some circumstances, patients can take a dose of as much as 180mg once a day.

Children aged between six and 12 years old can take up 30mg as needed, up to twice a day.

Children between the ages of four and six are not recommended to take this drug unless specifically advised to do so by a doctor. If the doctor recommends the drug then they will also recommend a dosage according to the particular patient.

This drug is not recommended to give to children under the age of four years old.

Loratadine (tablets or liquid)

Adults and children over the age of six can take up to 10 milligrams of Loratadine in single dose each day.

Children between the ages of four and six can take up to five milligrams once a day.

This drug is not recommended to give to children under the age of four years old.

Phenindamine (tablets)

Adult patients over the age of 12 can take doses of up to 25 milligrams with a minimum of four hours between each dose.

Children aged between six and 12 can take half of the adult dose (12.5 milligrams) of phenindamine as required with a minimum of four hours between doses.

Children under six should only take this drug if this is advised by a doctor.

This drug is not recommended to give to children under the age of four years old.

Missed doses

Missing a dose of antihistamines is not likely to cause any serious harm to patients, but they will not benefit from the relief of any symptoms related to allergic reactions. Patients who miss a dose of any of the medications listed above should take the next dose as soon as they remember. If it is nearly time for the next dose when they remember that they have missed one, the next dose should be taken instead and no effort should be made to catch up. Patients should never double up on doses to make up for a missed dose.

Overdose

Whilst antihistamines are generally regarded as safe to use by most people, it is still possible to bring on negative effects if they are taken in high quantities.

High quantities of antihistamines can cause profound sedation, abnormally fast heart rates and can bring on seizures. The danger of all of these effects is even greater for children.

In order to avoid overdose, it is extremely important to follow the points below:

  • NEVER double up on a dose
  • NEVER exceed the stated dose without first consulting your doctor
  • NEVER take a dose sooner than you are supposed to. Always wait until the next dose is due.
  • NEVER take more than one antihistamine at the same time unless specifically told to by your doctor
  • NEVER leave medication within easy reach of children

Interactions

Antihistamines are available over the counter at most pharmacies. They are not a controlled drug as they are generally regarded as being very safe to take for the majority of people, irrespective of other medications they might be taking or whether or not they suffer from an existing medical condition.

However, although the chances are small, as with all drugs there is always the possibility that two different medications may interact with each other. Such an interaction could lead to an increase in the quantity or severity of side effects experienced, either or both of the drugs operating less effectively or the development of an entirely different medical condition.

You will easily be able to purchase antihistamines without a prescription, however, you should seek medical advice before taking them if any of the following statements are true:

  • You are pregnant or plan to become pregnant whilst taking the medication
  • You are purchasing the antihistamines for a young child
  • You are taking any other regular medication, prescribed or prescribed
  • You have an existing heart, liver or kidney condition
  • You suffer from epilepsy or have previously had seizures
  • You have been diagnosed with Glaucoma
  • You currently have or have recently had an intestinal obstruction
  • You have a stomach ulcer

In some of the cases listed above, it may not be medically advisable to take antihistamines. It may be the case that a medical professional would advise you to take one type of antihistamine in particular as although they do the same job, different pharmaceuticals will interact with other drugs or medical conditions in different ways.

One of the most common side effects of antihistamines is drowsiness. If patients are already taking a medication which causes sedation, this effect can be heightened by taking antihistamines at the same time, causing a profound drowsiness making it extremely hard to function on a daily basis. Such medications include (but are not limited to):

  • Sleeping pills
  • Muscle relaxants
  • Some analgesics
  • Narcotics (both prescribed and recreational)
  • Antipsychotic medication
  • MAOIs (mood disorder medications)
  • Different antihistamines
  • Antidepressants

In addition to this list of medications, the chemicals in certain types of food and drink can also interact with antihistamines. For this reason, it is not recommended to eat grapefruit or to drink grapefruit juice whilst taking antihistamines. Alcohol and recreational drugs can also increase the drowsiness experienced by patients taking antihistamines.

Warnings

Allergic reactions

It is very uncommon for people to be allergic to antihistamines, however, it does happen. If you know that you have had an allergic reaction to an antihistamine, make sure that you consult a doctor before trying a different drug. If you are buying antihistamines without a prescription, make sure to read the label carefully and to check for other ingredients you are allergic to, such as food dyes, preservatives or chemicals.

Children and Antihistamines

Antihistamines are more likely to bring on serious side effects in children than they are in adults. Although it is still rare for children to experience these symptoms, there is a much higher chance that a child will develop seizures, an abnormal heart rate and/or difficulty breathing following the consumption of antihistamines than it is for adult patients. The younger the child, the higher the risk. Therefore it is only advisable to give your child antihistamines if they have been recommended by a doctor first. Under no circumstances should a child under the age of 4 be given antihistamines which are purchased over the counter. Antihistamines can endanger the lives of infants and small children.

Elderly patients and antihistamines

In the same way that children can be more sensitive to the side effects of antihistamines, so too can geriatric patients. Elderly patients are more likely than others to experience confusion, difficulty in passing urine, drowsiness, nervousness, nightmares, dryness in the throat, mouth and nose.

Antihistamines and pregnancy

Some antihistamines are not recommended to be taken in the early stages of pregnancy, as high doses of these drugs have in some cases been linked with birth defects in animal studies. If you are pregnant it is advisable to speak to a doctor before taking antihistamines in order to rule out the possibility of causing harm to the unborn baby.

Antihistamines and Breast Feeding

Babies are much more susceptible to the potential side effects of antihistamines than children and adults are. It is possible for small amounts of the antihistamine medication to pass from mother to baby through the breast milk, and as even small amounts can trigger harmful responses in babies, it is not advisable to take antihistamines whilst breast feeding.

Another complication with taking these drugs whilst breast feeding is that antihistamines are associated with inhibiting the body’s ability to secrete liquids, whether this be saliva, mucus, urine or breast milk. Lactation may be compromised in patients taking antihistamines whilst breast feeding.

Storage

All medications should be stored in their original packaging, with a clearly legible label so that they cannot be confused with anything else. Information leaflets and dosage instructions should be kept with the medication at all times so that they can be easily accessed if required. Medications should be kept out of the reach of children and pets so that they cannot take them by accident.

For different types of medication, there are different things to be aware of when finding a suitable place to store them.

Liquid-filled capsules should be kept well above freezing temperatures. If the liquid inside freezes, it will expand and burst the protective coating of the capsules. This will make the medication unusable and could cause contamination if the liquid comes into contact with food or other medications.

Liquid medications such as syrups should be stored in child-proof bottles with clear labels. They should be kept at room temperature and should not be allowed to freeze.

Oral tablets, powder-filled capsules and suppositories should be kept in their original blister packs and should be stored away from extremes in temperature. They should be kept in dry conditions so that they cannot be contaminated by moisture.

Antihistamines for injection are not commonly administered in the home and are therefore unlikely to require storage at home. If injections are required at home, then it is extremely important to follow the doctor’s instructions with regards to safe storage of the drugs. Needles should always be stored with great care, safely out of reach of anyone who could cause harm with them. Sharps bins must be provided so that used syringes can be disposed of without risk to anyone else.

Once medication is no longer required, it should be disposed of responsibly. Do not place into usual household waste, but instead return any unused, unwanted or out of date medication to a healthcare institution so that they can safely dispose of the medication.

Summary - MAX 700 words

Antihistamine is a term which refers to a type of medication most commonly used to relieve the symptoms of allergies, specifically by limiting the body’s production of histamine. There are many different types of drug which come under this category including: Azatadine, Brompheniramine, Cetirizine, Chlorpheniramine, Clemastine, Cyproheptadine, Desloratadine, Dexchlorpheniramine, Diphenhydramine, Doxylamine, Fexofenadine, Loratadine and Phenindamine.

When they were first developed, antihistamines, whilst effective at keeping allergies under control, had the negative side effect of causing pronounced drowsiness in people who took them. This could often be debilitating and limited the types of activity which patients could carry out whilst the drugs were in their system. These particular medications are now known as first generation antihistamines and are no longer commonly prescribed. A second generation of antihistamines was created to combat this problem and these are the ones which are commonly used today. The majority of them are much less likely to induce the side effect of drowsiness in patients.

Antihistamines are widely regarded as being safe for the vast majority of patients. They are available to purchase over the counter and without a prescription as they are not thought of as dangerous enough to require controlling. That being said, they can still provoke side effects in patients and so should be treated with caution.

The dosage required to relieve the symptoms associated with allergies entirely depends on the particular drug and the strength of its active ingredient. These differ greatly from one type of antihistamine to another, so patients are advised to read labels carefully before taking a dose. Children and elderly patients are much more susceptible to the side effects which can be experienced whilst taking the drug so it is generally advised to speak to a doctor before either of these types of patient takes the drug.

The side effects which can be associated with antihistamines are very rarely serious, but they can induce seizures in people who are already susceptible.

Resources
Last Reviewed:
December 24, 2017
Last Updated:
February 09, 2018
Content Source: