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Antihistamines: Definition, Types, Uses, Side Effects, Contraindications
Annotation or definition
Side Effects
Types of Antihistamines - The Antihistamine Directory
What are antihistamines?
What conditions are antihistamines used to treat?
How do antihistamines work?
Are there different types of antihistamines?
Which is the best one?
How do I take antihistamines?
How quickly do antihistamines work?
How long is treatment needed?
Who cannot take antihistamines?
What about side-effects?
Annotation or definition

What are antihistamines?
Antihistamines are defined as a class of drugs that block the physiological action of the chemical histamine in the body to neutralize the histamine induced allergic reactions and therefore, they form the main component in the treatment of allergy.


Antihistamines form the mainstay of all anti-allergy treatments playing an important role in getting rid of your allergy symptoms and giving you relief.

They are thus used in the treatment of:
•Seasonal allergic rhinitis (hay fever)
•Allergic sinusitis
•Cough due allergic bronchitis
•Bronchial asthma
•Watery and itchy eyes
•Itching in the nose and the throat
•Allergic skin conditions such as itching (pruritis), skin rash, hives (urticaria), eczema and psoriasis
•Insect bites and bee stings
•Stomach upset and pain due to food allergy
•Antihistamines cause drowsiness and are, therefore, used to induce sleep in treating insomnia
•As a supplement in the treatment of motion sickness
•In the treatment of anaphylactic shock

Antihistamines are available as tablets or pills, capsules, liquids, nasal sprays, eye drops and as injectables. They are prescribed twice or thrice a day depending on the severity of the hypersensitivity attack.

OTC antihistamine examples

Some of the antihistamine medicines are available over the counter (OTC) while some need a prescription. The list is long but to name a few with their brand names:

Over the counter antihistamines examples include:
•Brompheniramine (Dimetane)
•Diphenhydramine HCl (Benadryl)
•Chlorpheniramine maleate (Chlor-Trimeton)
•Fexofenadine (Allegra)
•Cetirizine (Zyrtec)

Prescription antihistamines include
•Azelastin nasal spray (Astelin)
•Azelastin eyedrops (Optivar)
•Carbinoxamine (Palgic)

Classification of antihistamines

Antihistamines are classified into two groups:

The first generation antihistamines
The first generation antihistamines are those, which include the older ones and which cause drowsiness or sedation. They do this because they are able to cross the blood brain barrier and act on the histamine H1 receptors in the brain to block the action of histamine. They are, therefore, called H1 receptor antagonist or H1 blockers.
First generation antihistamnics can cause antimuscarinic side effects by which they relax the smooth muscles and cause decreased secretion of saliva, sweat and the digestive juices.

Examples of first generation antihistamines


Side effects due to antimuscarinic properties include:
•dry mouth due to decreased secretion of the saliva
•blurred vision due to dilation of the pupil
•constipation due to relaxation of the smooth muscles of th
e bowels
•retention of urine due to relaxation of the smooth muscles of the urinary bladder

Not all first generation antihistamine drugs exhibit antimuscarinic or anticholinergic properties. For example Oxatomide is a first generation sedating antihistamine without antimuscarinic effects.
The second generation antihistamines
The second generation antihistamines were introduced to remove the side effect of drowsiness found with the older first generation antihistamines.
They keep the patient alert because they do not cross the blood-brain barrier to an extent as much as the older antihistamines did. To a great extent, this was successful in most people, but some people did experience some drowsiness.
The second generation type of antihistamines last in the blood for a longer time and if taken in higher doses they can induce drowsiness.
Terfenadine and astemizole were the first second generation antihistamines to be introduced, but they produced side effects of ventricular arrhythmias. They were withdrawn from production.
Currently, mequitazine, cetirizine, loratadine, mizolastine, fexofenadine are marketed and they do not produce sedation because they do not cross the blood brain barrier. Their action is and remains peripheral.
Of those mentioned above, mequitazine exhibits antimuscarinic properties and you can therefore experience side effects of these properties, mentioned above.

Contraindications for antihistamines

People with certain conditions may not be advised antihistamines. These conditions include:
•a type of metabolic disorder (acute porphyria)
•during pregnancy
•during lactation
•prostate enlargement (benign prostatic hyperplasia)
•acute glaucoma, a condition where the pressure in the eye is raised
•when you’re trying to conceive

Drug interactions

Antihistamines are to be used with caution and close monitoring if you are taking antidepressants and/or anti-fungal drugs. This is when you should seek your doctor’s opinion.

Antihistamines and alcohol

Since first generation antihistamines cause drowsiness, visual disturbances and heart palpitations, consuming alcohol when you are under the effect of antihistamines, can increase the risk and depth of these side effects.
Second generation antihistamines do not cause drowsiness and consuming alcohol while on these drugs may not increase sedation, but there are other side effects like heart problems due to drug interaction, that contraindicates taking alcohol while on these drugs.
However, some experts feel that if you are taking the second hand generation antihistamines within the prescribed dose, there is no harm if you take alcohol.
The bottom line is to restrict the amount of alcohol to the prescribed level of not more than two drinks (60 ml equals one drink), drunk well diluted and over time. This will keep the level of alcohol in the blood to well below the dangerous levels to cause any dangerous drug interaction with the antihistamines.
Do antihistamines raise blood pressure?
Antihistamines can safely be used in patients with high blood pressure to relieve the symptoms of allergy, because they do not cause any effect on the blood pressure.
Nasal corticosteroids too can be used to relieve the nasal congestion.
However, decongestants, which are also used to relieve allergy discomfort are contraindicated in patients with high blood pressure, rhythm problems (palpitations), or who have severe blockages in their blood vessels to their heart. They can increase blood pressure and can accelerate heart rate.
Therefore, though you can use Allegra tablet for your allergy, you should not use Allegra-D, which is an antihistamine combined with a decongestant.

Anti-Inflammatory Agents
Drugs Used to Treat Inflammation
What are anti-inflammatory painkillers?
When are anti-inflammatory painkillers used?
How do anti-inflammatory painkillers work?
What are the possible side-effects and risks?
Prescription Nonsteroidal Anti-Inflammatory Medicines
How do prescription nonsteroidal anti-inflammatory drugs work?
What are some common prescription NSAIDs?
What's the main difference between traditional NSAIDs and COX-2 inhibitors?
Do prescription NSAIDs have any side effects?
What is a drug interaction?
Drugs Used to Treat Inflammation

Nonsteroidal Anti-inflammatory Drugs
Chondroprotective Agents

NSAID's, or non-steroidal anti-inflammatory drugs
Steroids/anti-inflammatory agents

How do prescription nonsteroidal anti-inflammatory drugs work? Nonsteroidal anti-inflammatory drugs (also called NSAIDs) stop cyclooxygenase enzymes (also called COX enzymes) in your body from working. COX enzymes speed up your body's production of hormone-like substances called prostaglandins. Prostaglandins cause the feeling of pain by irritating your nerve endings. They are also part of the system that helps your body control its temperature. By reducing the level of prostaglandins in your body, NSAIDs help relieve pain from conditions like arthritis. They also help reduce inflammation (swelling), lower fevers and prevent blood from clotting. What are some common prescription NSAIDs? There are 2 classes of prescription NSAIDs. Traditional NSAIDs include the following: •Diclofenac •Etodolac •Fenoprofen •Flurbiprofen •Ibuprofen •Indomethacin •Meclofenamate •Mefenamic Acid •Meloxicam •Nabumetone •Naproxen •Oxaprozin •Piroxicam •Sulindac •Tolmetin COX-2 inhibitors include celecoxib. If you need to take a prescription NSAID, your doctor will help you find one that is right for you. What's the main difference between traditional NSAIDs and COX-2 inhibitors? You have 2 types of COX enzymes in your body: COX-1 and COX-2. Researchers believe that one of the jobs of COX-1 enzymes is to help protect your stomach lining. The COX-2 enzyme doesn't play a role in protecting your stomach. Traditional NSAIDs stop both COX-1 and COX- 2 enzymes from doing their jobs. When COX-1 enzymes are blocked, pain and inflammation is reduced, but the protective lining of your stomach is also reduced. This can cause problems such as upset stomach, ulcers, bloating and bleeding in your stomach and intestines. COX-2 inhibitors only stop COX-2 enzymes from working. Since the COX-2 enzyme doesn't help to protect your stomach, COX-2 inhibitors may be less likely to irritate your stomach or intestines. Do prescription NSAIDs have any side effects? Like all medicines, these drugs can cause side effects. However, the side effects usually are not severe and are not experienced very often. Common side effects of prescription NSAIDs may include the following: •Dizziness •Headache •Nausea •Diarrhea •Excess gas •Constipation •Extreme weakness or fatigue •Dry mouth Serious side effects of prescription NSAIDs may include the following: •Allergic reaction, such as difficulty breathing, hives, swelling of the lips, tongue or face •Muscle cramps, numbness or tingling •Rapid weight gain •Black, bloody or tarry stools •Bloody urine or bloody vomit •Decreased hearing or ringing in the ears (also called tinnitus) •Jaundice (the yellowing of the skin and the whites of the eyes) •Abdominal cramping •Heartburn •Indigestion In addition to the side effects listed above, people taking a COX-2 inhibitor may be at risk for the following side effects: •Swelling or water retention •Skin rash or itching •Unusual bruising or bleeding •Difficulty sleeping (insomnia) Call your doctor as soon as possible if your side effects become severe. Is it safe to take NSAIDs for a long period of time? People who take NSAIDs increase their risk of developing severe bleeding in their stomachs. They may also be at risk for heart attacks and strokes. These risks gets worse if they take higher doses and/or if they take these medicines for a long period of time. Patients who need to take pain medicine for longer than a week should discuss this risk and explore other pain treatment options with their family doctor. What is a drug interaction? If you use 2 or more drugs at the same time, the way your body processes each drug can change. When this happens, the risk of side effects from each drug increases and each drug may not work the way it should. This is called a drug-drug interaction. Vitamins and herbal supplements can affect the way your body processes drugs, also. Certain foods or drinks can also prevent your medicine from working the way it should or make side effects worse. This is called a drug-food interaction. For example, if you're taking a traditional NSAID, drinking alcohol can increase your risk of liver disease or stomach bleeding. Drug-drug interactions and drug-food interactions can be dangerous. Be certain that your doctor knows all of the over-the-counter and prescription medicines, vitamins and herbal supplements that you are taking. Also, talk to your doctor before you take any new over-the-counter or prescription medicine or use a vitamin or herbal supplement. It's important to take medicines exactly as your doctor prescribes. Ask your doctor whether you need to avoid any foods or drinks while using a prescription NSAID.


Nonsteroidal anti-inflammatory drugs (NSAIDs) you can buy without a prescription (not a complete list)

Generic Name Brand Name
ibuprofen Advil, Motrin
naproxen Aleve

Aspirin is another example of an NSAID that you can buy without a prescription. But aspirin works differently from these other NSAIDs. See the topic Aspirin for more information.

Prescription NSAIDs (not a complete list)

Generic Name Brand Name
celecoxib Celebrex
diclofenac Voltaren
naproxen Anaprox
piroxicam Feldene
sulindac Clinoril

Steroids/anti-inflammatory agents

Generic Name   
Common Name
Removed from Market
Amcinonide Cyclocort  
Betamethosone diproprionate Diprosone  
Clobetasol Click here for a list of names  
Clocortolone Click here for a list of names  
Dexamethasone Decadron  
Diflorasone Click here for a list of names  
Dutasteride Click here for a list of names  
Flumethasone Pivalate Locorten  
Flunisolide Click here for a list of names  
Fluocinolone Acetonide Click here for a list of names  
Fluocinonide Click here for a list of names  
Fluorometholone Click here for a list of names  
Fluticasone propionate Advair Diskus  
Fluticasone propionate Flonase  
Fluticasone propionate Flovent  
Flurandrenolide Click here for a list of names  
Hydroflumethiazide Click here for a list of names  

Antihistamines (Oral)

What types of OTC antihistamines are available?

How do antihistamines work?

How do I safely take OTC antihistamines?

What are some common side effects of OTC antihistamines?

Could OTC antihistamines cause problems with any other medicines I take?

Who shouldn’t take antihistamines?

What types of OTC antihistamines are available?
Over-the-counter (OTC) medicines are medicines you can buy without a prescription from your doctor. Antihistamines help relieve or prevent allergy symptoms. Two types of OTC antihistamines are available: first-generation and second-generation antihistamines. Both types can be useful for allergies. First-generation antihistamines are also sometimes used in OTC cold medicines.

How do antihistamines work?
When your body is exposed to allergens, it releases histamines. Histamines attach to the cells in your body and cause them to swell and leak fluid. This can cause itching, sneezing, runny nose, and watery eyes. Antihistamines prevent histamines from attaching to your cells and causing symptoms.

First-generation antihistamines also work in the part of the brain that controls nausea and vomiting. This is why they can help prevent motion sickness. Because 1 of the most common side effects of first-generation antihistamines is feeling sleepy, they are sometimes used to help people who have trouble sleeping (insomnia).

First-Generation OTC Antihistamines
•Brompheniramine (1 brand name: Dimetapp Cold and Allergy Elixir)
•Chlorpheniramine (1 brand name: Chlor-Trimeton)
•Dimenhydrinate (1 brand name: Dramamine)
•Diphenhydramine (2 brand names: Benadryl Allergy, Nytol, Sominex)
•Doxylamine (2 brand names: Vicks NyQuil, Alka-Seltzer Plus Night-Time Cold Medicine)
Second-Generation OTC Antihistamines
•Loratadine (2 brand names: Alavert, Claritin)
•Cetirizine (1 brand name: Zyrtec)
•Fexofenadine (1 brand name: Allegra)
Note: Both types of antihistamines often are mixed with other medicines, such as pain relievers or decongestants. Many of the brand names above are for these combination medicines, which are meant to treat many symptoms at the same time. In general, it’s a good idea to treat just the symptoms that you have. For example, if you have only a runny nose, don’t choose a medicine that also treats headache and fever.

How do I safely take OTC antihistamines?
Read the directions on the drug facts label to learn how much medicine to take and how often to take it. If you have any questions about how much medicine to take, call your family doctor or pharmacist. Keep a record (1-page PDF; About PDFs) of the OTC medicines you are using and when you take them. If you need to go to the doctor, take this list with you.

Follow these tips to make sure you are taking the right amount of medicine:

•Take only the amount recommended on the medicine’s label. Don’t assume that more medicine will work better or quicker. Taking more than the recommended amount can be dangerous. •If you are taking a prescription medicine, ask your doctor if it’s okay to also take an OTC antihistamine.
•Don’t use more than 1 OTC antihistamine at a time unless your doctor says it’s okay. They may have similar active ingredients that add up to be too much medicine. How can I safely store OTC antihistamines?
Store all medicines up and away, out of reach and sight of young children. Keeping medicines in a cool, dry place will help prevent them from becoming less effective before their expiration dates. Do not store medicines in bathrooms or bathroom cabinets, which are often hot and humid.

What are some common side effects of OTC antihistamines?
Healthy adults don’t usually experience side effects from antihistamines. However, side effects can be a concern for older adults or people who have health problems.

First-generation antihistamines may make you feel sleepy. This can affect your ability to drive or operate machines, and it may be hard for you to think clearly. Alcohol can increase the drowsiness caused by antihistamines. Antihistamines may cause your mouth and eyes to feel dry. They can also cause abdominal pain and headaches. Second-generation antihistamines are less likely to cause these side effects.

Could OTC antihistamines cause problems with any other medicines I take?
Antihistamines can interact with other medicines you take. If you take any of the following medicines, talk to your doctor before taking a first-generation antihistamine:

•Sleeping pills
•Muscle relaxants
Antihistamines are often combined with decongestants and/or pain relievers. If you take 1 of these combination medicines, it’s important to understand each of the active ingredients and the interactions they may have with other medicines you’re taking.

Be sure not to take too much antihistamine. Many OTC cold and allergy medicines contain antihistamines, and some prescription medicines do, too. If you take more than 1 of these medicines, you may get much more antihistamine than you intend.

Second-generation antihistamines are less likely to interact with other medicines you are taking.

Who shouldn’t take antihistamines?
Talk to your doctor before using a first-generation antihistamine if you have any of the following health problems:

•Trouble urinating (from an enlarged prostate gland)
•Breathing problems, such as asthma, emphysema, or chronic bronchitis
•Thyroid disease
•Heart disease
•High blood pressure

Questions for your doctor

Patients may wish to ask their doctor the following questions related to antihistamines:
Are antihistamines safe for me to take?
What is your opinion about using antihistamine medications to treat cold symptoms?
How and when should I take this drug?
What side effects may I develop?
For what side effects should I seek medical attention?
How long will I have to wait before the drug to takes effect?
How long will I have to take this drug?
Are there medications, nutritional supplements or herbal medications I should avoid while taking this drug?
How will I know if the drug is working?
What are other possible treatments if the drug does not work?

Advil PM
Advil PM contains diphenhydramine (antihistamine/anticholinergic) and ibuprofen (NSAID). Exactly how the NSAID works is not known. It may block certain substances in the body that are linked to inflammation. NSAIDs treat the symptoms of pain and inflammation. They do not treat the disease that causes those symptoms. The antihistamine/anticholinergic works in the brain to cause sedation.
Assured Multi-Symptom Day & Night Cold Formula, 12-ct.
Product Details

Active ingredients in cough and cold medicines
Nighttime formula
Acetaminophen 325mg acetaminophen (pain reliever/fever reducer)
Chlorpheniramine maleate (antihistamine) 2mg chlorpheniramine maleate (antihistamine)
Dextromethorphan 10mg dextromethorphan HBr (cough suppressant)
Phenylephrine 5mg phenylephrine HCI (nasal decongestant) per caplet
Non-drowsy daytime formula
Acetaminophen 325mg acetaminophen (pain reliever/fever reducer)
Dextromethorphan 10mg dextromethorphan HBr (cough suppressant)
Phenylephrine 5mg phenylephrine HCI (nasal decongestant) per caplet
Relieve all your symptoms from colds in the day or nighttime with multi-symptom cold formula cool caplets! Nighttime formula has 325mg acetaminophen (pain reliever/fever reducer), 2mg chlorpheniramine maleate (antihistamine), 10mg dextromethorphan HBr (cough suppressant), and 5mg phenylephrine HCI (nasal decongestant) per caplet. Non-drowsy daytime formula has 325mg acetaminophen (pain reliever/fever reducer), 10mg dextromethorphan HBr (cough suppressant), and 5mg phenylephrine HCI (nasal decongestant) per caplet. Case includes a counter display plus 36 – 12-ct. boxes of Assured™ Multi-symptom Day & Night Cold Formula (6 daytime cool caplets, 6 nighttime cool caplets per box).
Last Updated: October 10, 2016