Antihistamines are the most commonly used type of drug for the treatment of allergy symptoms. The drugs work by blocking the effects of a chemical called histamine, which is released into the bloodstream during allergic reactions.

By using an antihistamine, an individual is able to reduce many of the symptoms commonly associated with allergies, including runny nose, watery eyes, itchiness and an irritated throat. Antihistamines are also used in some cases to treat motion sickness, sleeplessness and even Parkinson’s disease.

Antihistamines are sold under a variety of generic and brand names, and are available in both over-the-counter and prescription form. Depending on the type of antihistamine, the drugs can be administered through several different methods, including:

• Pills
• Liquid
• Nasal sprays
• Eye drops
• Topical creams

Children and the elderly population should be careful when using antihistamines as side effects can be more pronounced among these two groups. Potential side effects include drowsiness, irritability and nightmares.

Pregnant mothers can generally take one category of antihistamine – known as category B – with a very low risk of complications, though they should always contact a physician before taking any type of medication.

Antihistamines are a class of medications used to counteract the effects of histamine – a chemical released during an allergic reaction. They relieve the symptoms associated with allergies including runny nose, watery eyes, itchiness and an irritated throat. The most commonly used type of allergy medication, antihistamines are available in many different forms and come in both over-the-counter and prescription strengths.

Antihistamines work by blocking the histamines released by body cells (mast cells and basophils). During an allergic reaction, IgE antibodies in the bloodstream trigger mast cells to release powerful chemicals when they come into contact with an allergen. These chemicals are designed to allow the body to fight off the allergen, which the body has mistaken as a threat. Most allergy symptoms are caused by the cellular swelling and release of fluids caused by the histamines and other chemicals.

As histamine flows through the body, it irritates nerve endings, causing itchiness. Histamines in the gastrointestinal tract can cause diarrhea, fluid secretion and cramps. When histamines interact with muscles in the bronchial airways, the airways can tighten and restrict breathing.

Histamines affect cells by interacting with the receptors on the cell’s surface. Antihistamines have a molecular structure that resembles that of histamine. They therefore block the cell receptors that would usually accept histamines. This essentially makes the histamine molecules into ships with nowhere to dock. With the effects of the histamines interrupted, the severity of allergy symptoms is reduced.

Since antihistamines work best when they can move in and block histamines before they arrive at a cell, it is useful to take an antihistamine treatment before symptoms appear. Older types of antihistamine need about 30 minutes to take effect. Newer, nonsedating antihistamines may need an hour to reach full effect, though some may require several days of use to achieve full effect. It is important to follow the physician's dosage recommendation. A physician might prescribe antihistamines to be taken a week or two before the beginning of hay fever season, for instance, and to continue taking the medication even if symptoms are not present.

A physician can recommend different over-the-counter or prescription antihistamines based on a patient's symptoms. For instance, if symptoms are primarily nasal congestion, then a nasal antihistamine may be recommended. Certain antihistamines may cause certain side effects in some people, so a patient may need to try different brands.

There are two types of antihistamine receptors and medications may be targeted toward blocking one or the other:

• Histamine 1 (H1) receptors. These are located in many body tissues, including the capillaries (small blood vessels). They are involved with most allergic rhinitis (hay fever) reactions. Medications designed to block these receptors are called H1 blockers and are the most commonly used antihistamines.
  
  
• Histamine 2 (H2) receptors. H2 receptors are located in the lining of the stomach. Medications designed to block these receptors are called H2 blockers. H2 blockers were originally developed to treat stomach ulcers, though they have recently been used in combination treatments with H1 blockers to treat hives.

It is important to note that antihistamines can cause falsely negative allergy skin test results. Therefore, patients will need to stop taking the drug some time before the test. The time needed to clear antihistamines from the body before testing should be discussed with a physician.

Some common antihistamine medications include:

Antihistamines are administered though several types of methods, including:

• Tablet. Solid form of medication that is swallowed. The medication dissolves and is absorbed into the bloodstream through the lining of the stomach and intestines.
  
  
• Liquid. Liquid form of medication that is swallowed. The medication is already dissolved and therefore is absorbed more quickly into the bloodstream through the lining of the stomach and intestines.
  
  
• Nasal spray. Sprays that distribute medicine – through the nose – directly to the nasal passages in the form of a fine mist.
  
  
• Eye drop. A sterile solution or medicine that is applied directly to the surface of the eye in the form of liquid drops.
  
  
• Topical cream. Medications in lotion or ointment form that can be spread directly onto the skin where it is absorbed into the body.

Antihistamines are often divided into the following groups:

• Nonsedating antihistamines. Because of the sleepiness experienced by almost half of those people taking early forms of antihistamine, many were unable to use this group of drugs. As a result, nonsedating (non–drowsy) forms of this medication were developed. Although early forms of nonsedating antihistamines were found to be unsafe, newer types (e.g., loratadine and fexofenadine) are considered both safe and effective at providing allergy relief without sedation. The early, unsafe types of antihistamines have all been removed from the market in the United States.
  
  
• Lightly sedating antihistamines. Some forms of newer antihistamines are found to cause sedation in only about 10 percent of the individuals who use the drug. For this reason, the U.S. Food and Drug Administration has decided that these forms of antihistamines – such as cetirizine – should by called “lightly sedating” instead of “nonsedating.”
  
  
• H1 antihistamines. These antihistamines are used to block the H1 receptors present in many types of tissue, thus preventing an allergic response. Because H1 receptors are the most common type of receptor, this is the most common and frequently used form of antihistamine. It is often used to effectively treat the symptoms of hay fever.
  
  
• H2 antihistamines. This type of antihistamine has fewer applications than H1 antihistamines because of the relative lack of H2 receptors in the body. Most H2 receptors are located in the stomach, making H2 antihistamines mainly effective at treating conditions such as stomach ulcers. This type of antihistamine has recently been used in combination therapies with H1 antihistamines to successfully treat conditions such as hives.

Antihistamines are prescribed for a variety of conditions, including:

• Allergies. Most allergy symptoms, including runny nose and sneezing can be effectively treated by taking an antihistamine before or during the reaction. Allergies are triggered by a variety of allergens (e.g., pollen, dander, mold, food). Conditions caused by allergic reactions to these substances are all treated with antihistamines. These include:
  
  
• • Allergic rhinitis. Also known as hay fever, antihistamines are effective at treating the congestion and itchy eyes associated with this condition.
    

    
    

  • Allergic conjunctivitis. This inflammation of the tissue on the inside of the eyelid can be reduced using antihistamines.

    

  • Allergic sinusitis. Antihistamines can reduce the inflammation of the lining of the sinus cavities that occurs with sinusitis.
    
    

    

  • Hives. These red, swollen patches of skin that often itch or burn can be treated with an antihistamine.
    

    

    
    

  • Anaphylactic shock. This potentially life-threatening condition involves difficulty breathing and a drop in blood pressure as the result of exposure to an allergen. These severe reactions typically require a shot of epinephrine to treat symptoms; however physicians may recommend that patients take antihistamines following an injection of epinephrine. Although antihistamines cannot stop an anaphylactic reaction, they can reduce the severity of symptoms. Patients prone to anaphylaxis may be instructed by their physician to include antihistamines in their allergy kit. Individuals who are susceptible to anaphylaxis and unresponsive to epinephrine may also find antihistamines useful. However, antihistamines cannot reverse anaphylaxis and are not a substitute for epinephrine.
    
     
• Atopic dermatitis. This is an itchy inflammation of the skin, often characterized by flaking. Histamine levels appear elevated in the skin and plasma of some individuals with this condition, making an antihistamine a useful treatment.
  
  

  

• Contact dermatitis. Inflammation that occurs on the skin’s surface after coming into contact with an allergen or irritant. Over-the-counter antihistamine lotions may be applied directly to the skin to relieve itching associated with contact dermatitis. Prescription antihistamines may be used if over-the-counter antihistamines do not relieve the itching.  
  
  
• Angioedema. A condition in which the skin swells and welts form on the skin, usually near the eyes, mouth, hands, feet and genitals. Over-the-counter antihistamines may help relieve minor symptoms of angioedema.  
  
  
  
  
• Cold and flu symptoms. These include coughing, runny nose and watery eyes.
  
  
• Sleeplessness. The drowsiness caused by many types of antihistamine can be used as an effective sleep aid.
  
  
• Motion sickness, dizziness, nausea and vomiting. Antihistamines are able to block certain signals to the brain and can reduce these kinds of symptoms.
  
  
• Nervousness and anxiety (these symptoms are usually treated with doxepin or hydroxyzine).
  
  
• Parkinson’s disease. In these patients the drug will sometimes decrease stiffness and tremors.

Patients should not take antihistamines, unless prescribed by a physician, if they have been diagnosed with any of the following conditions:

• Asthma. Antihistamines can provoke an asthma attack in some individuals.

• Ear infection (otitis media). Antihistamines can make an ear infection worse and/or lengthen the time it takes to resolve.
  
  
• High blood pressure (hypertension). Decongestant and antihistamine combinations can elevate blood pressure. Individuals with hypertension should avoid both to be safe.
  
  
• Enlarged prostate, intestinal obstruction, stomach ulcer or urinary tract block. Antihistamine use can aggravate these conditions.
  
  
• Heart problems. Antihistamines can produce heart palpitations (an awareness of a strong, fast, irregular, abnormal or “galloping” heartbeat) or arrhythmias (abnormal heartbeats that may be unusually fast or unusually slow) in individuals with heart conditions.
  
  
• Thyroid conditions. Thyroid conditions can lead to an elevated heartbeat, which can be further complicated with antihistamine use.
  
  
• Liver, heart, lung or kidney disease. Antihistamines may cause increased enzyme production that can interfere with these conditions.
  
  
• Glaucoma (a group of eye diseases that affect the optic nerve, which connects the eye to the brain). Antihistamine use can aggravate glaucoma.

Some types of antihistamines produce slightly different side effects. For example, older forms of first-generation antihistamines are known to affect coordination and make it harder to concentrate. More recent versions no longer have this effect.

The side effects associated with antihistamines sometimes diminish after several days of use. However, a person should immediately contact a physician if they experience any of the following side effects to antihistamines:

• Low blood pressure
• Dizziness
• Inability to urinate
• Labored breathing
• Increased heartbeat
• Vomiting

Less serious side effects of antihistamines include:

• Drowsiness (more common with over-the-counter antihistamines)
• Dry mouth or a bitter taste in mouth
• Nausea
• Restlessness or irritability
• Blurred vision
• Constipation

Children or elderly persons taking antihistamines may experience more intense side effects or other conditions, such as:

• Seizures
• Confusion and nervousness
• Dryness of the nose, mouth or throat
• Difficult or painful urination
• Nightmares

Patients using antihistamines should consult their physicians before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal remedies. Of particular concern to individuals taking antihistamines are:

• Erythromycin. A type of antibiotic used to fight many types of bacterial infection. Use of this drug with an antihistamine can lead to an elevated amount of some antihistamines in the blood.
  
  
• Anticholinergics. Medicine used to treat abdominal and stomach spasms as well as cramps. Anticholinergics can cause drowsiness (and other side effects also associated with antihistamines) and therefore should not be combined with antihistamines.
  
  
• Monoamine oxidase (MAO) inhibitors. Medications used in the treatment of depression. They can cause severe drowsiness when taken with antihistamines.
  
  
• Central nervous system depressants. Used to treat anxiety, muscle tension, pain, insomnia, acute stress reactions, panic attacks and seizure disorders by slowing down brain activity. Using this drug with antihistamines can worsen the side effects associated with both drugs.
  
  
• Antifungals. Medicine used to treat fungal infections. Certain antifungals should not be used in combination with antihistamines. For instance, ketoconazole can lead to an elevated amount of fexofenadine in the blood when taken with the antihistamine. Itraconazole should also be avoided.
  
  
• Aspirin. Symptoms associated with the use of large amounts of aspirin can be masked when using antihistamines.
  
  
• Alcohol. Effects associated with the consumption of alcohol can be increased with the use of antihistamines.

Symptoms of overdose can be similar to the medication’s side effects, but are usually more severe. Patients exhibiting any of these symptoms should contact their physicians immediately:

• Unsteadiness, tremor or convulsions of the body
• Flushing or redness of the face
• Rapid heartbeat or low blood pressure
• Symptoms of the nervous system, including:
• Depression
• Nervousness
• Drowsiness
• Hallucinations
• Disorientation
• Delirium
• Seizures

For less severe symptoms that do not indicate overdose but require contacting a physician, see Potential side effects.

Pregnant women should always consult their physician before using any type of antihistamine. The risk of antihistamine use during pregnancy is based on the specific type of antihistamine being used. The U.S. Food and Drug Administration (FDA) publishes a comprehensive listing of risk categories for antihistamines, which includes risk factors to pregnant women. Only two categories of drugs, B and C, are currently available for use in the United States. The risk factors associated with these two categories are:

• Category B. There is no evidence in human studies that this category poses a risk to the fetus, and no animal studies have conclusively demonstrated that this category of antihistamines produces an adverse effect on the developing fetus. The FDA regards the risk associated with this category to be very low. Therefore, this is considered to be the safest type of antihistamine for use during pregnancy. This category includes chlorpheniramine, diphenhydramine, cetirizine and loratadine.
  
  
• Category C. While there are no human studies to rely on, animal studies have shown that this category of drugs can produce birth defects or the loss of pregnancy. This category of drugs includes fexofenadine and desloratadine.

While breastfeeding mothers should not be overly concerned with the use of antihistamines, they should still contact their physicians before taking the drug. The medication is passed through breast-milk, though it is in such a diluted form (one-hundredth of the original dose taken) that it should not represent a significant problem. While antihistamine use can lower the amount of milk produced by the new mother, this condition is not permanent.

There are several side effects to consider before giving a child an antihistamine. These include:

• Drowsiness. Antihistamines can make many children sleepy. For this reason it may be a good idea to give the drug to children at bedtime.
  
  
• Hyperactivity or jitteriness. Some children react to antihistamines with varying types of restlessness. Concerned parents should contact their child’s physician to arrange for a dosage or medication change.
  
  
• Seizures. This condition is more likely to occur in children who take antihistamines than in adults. If a child experiences convulsions, parents should alert their child’s physician immediately.
  
  
• Nightmares and irritability. These symptoms may appear in varying degrees in some children. Concerned parents can consult with their child’s physician to change drugs or dosage.

Parents are encouraged to discuss their concerns about antihistamine use with their child’s physician.

Elderly patients are typically more susceptible to the effects of antihistamines. For this reason, older adults taking antihistamines often experience:

• Drowsiness
• Confusion, nervousness or irritability
• Dizziness
• Dryness in the nose, mouth or throat
• Difficult or painful urination
• Nightmares

Preparing questions in advance can help patients to have more meaningful discussions with their physicians. Patients may wish to ask their doctors the following questions related to antihistamines:

1. Are antihistamines safe for me to take?
   
   
2. How and when should I take this drug?
   
   
3. What should I do if I miss a dose?
   
   
4. Would taking an antihistamine allow me to eat a food to which I am normally allergic?
   
   
5. What side effects may I develop?
   
   
6. For what side effects should I seek medical attention?
   
   
7. How long will it take for the drug to take effect?
   
   
8. How long will I have to take this drug?
   
   
9. Are there medications, nutritional supplements or herbal medications I should avoid while taking this drug?
   
   
10. How will I know if the drug is working?
    
    
11. What are other possible treatments if the drug does not work?