Antihistamines are drugs that block the effects of a chemical called histamine. Produced by the body, histamine can cause symptoms such as itching, sneezing, runny nose and watery eyes. Antihistamines are used to treat skin conditions such as itchy rashes. They are particularly effective in treating hives. In addition, antihistamines are the most commonly used type of drug for the treatment of allergy symptoms.

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 in several different forms, including:

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

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

Antihistamines are a class of medications used to counteract the effects of histamine. This chemical, which is produced by the body, can cause a number of symptoms including:

• Itching (pruritus)
• Sneezing
• Runny nose
• Watery eyes

Antihistamines are sometimes used to treat skin conditions, such as itchy rashes. In addition, antihistamines are the most commonly used form of allergy medication.

Antihistamines are used to interrupt a natural reaction in the body. As histamine flows through the body, it irritates nerve endings, causing itchiness. 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 itching and other symptoms is reduced.

Some common antihistamine medications include:

Antihistamines are administered though several types of methods, including:

• Topical cream. Medications in lotion or ointment form that can be spread directly onto the skin where they are absorbed into the body.
  
  
• 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.

Some medications combine antihistamines with other drugs (e.g., decongestants) to provide relief for a wider range of symptoms.

There are two types of antihistamine receptors and medications may be targeted toward blocking either type. Histamine 1 (H1) receptors 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 type of antihistamines.

Histamine 2 (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.

In addition, antihistamines are divided into either nonsedating or sedating medications. Nonsedating antihistamines offer the benefits of antihistamines without making patients drowsy. 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 relief without sedation. The early, unsafe types of antihistamines have all been removed from the market in the United States.

Antihistamines are prescribed for a variety of conditions. In some cases, over-the-counter versions of these drugs may provide relief. In other cases, prescription strength formulations may be necessary. Patients should not take any medication without first consulting a physician.

Many skin conditions are treated with oral or topical antihistamines. These include:

• Atopic dermatitis. 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.
  
  

  

• Dry skin (xerosis). Antihistamine pills may be used to treat itchiness (pruritus) associated with dry skin.
  
  
• Hives (urticaria). Red, swollen patches of skin that often itch or burn. H1 blockers are the standard medication used to treat both acute and chronic urticaria. In some cases, these drugs may be combined with H2 blockers, oral corticosteroids or tricyclic antidepressants.
  
  
• Angioedema. A condition characterized by swelling and the formation of welts (raised marks) on the skin. It is similar to hives, except the welts are larger and form at a deeper layer in the skin. Over-the-counter antihistamines may help relieve minor symptoms such as itching and swelling.
  
  
• Lichen planus. Skin disease that causes purple or reddish-purple bumps on the skin that have flat tops. Most commonly found on the inner wrists, forearms and ankles.
  
  
• Pityriasis rosea. Acute, self-limiting, inflammatory skin disease that tends to occur in adolescents and young adults, especially in the spring and fall. This scaly, reddish-pinkish rash tends to appear as a single spot on the stomach or back. Later, additional spots may appear.
  
  
• Allergic contact dermatitis. A localized rash or skin inflammation caused by direct contact with an allergen (a substance that triggers an exaggerated immune system response). Antihistamines may be used to treat rashes that occur after exposure to allergens. For instance, itchiness associated with allergic reactions due to contact with poison ivy, oak and sumac may be treated with antihistamines.
  
  
• Pruritus. The medical name for generalized itching. In some cases, antihistamines can help relieve this itching. However, there are cases where nonsedating antihistamines are of little value in relieving symptoms, such as pruritus related to atopic dermatitis. In such cases, systemic, sedating antihistamines may be required to control symptoms.
  
  
• Scabies. An intensely itchy skin condition caused by a tiny mite that infests the skin. Antihistamines are used to treat this itch, which can last for weeks after the mite has been removed.
  
  
• Insect bites and stings. Antihistamines can help relieve the pain, swelling and itchiness associated with some insect bites and stings.

In addition to skin conditions, antihistamines are the chief treatment for many allergic conditions, such as allergic rhinitis (hay fever). Antihistamines also are used to treat the following:

• Cold and flu symptoms
  
  
• Motion sickness
  
  
• Nervousness
  
  
• Parkinson’s disease (progressive nervous disease associated with the destruction of brain cells and characterized by symptoms such as muscular tremor)
  
  
• Sleeplessness

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

• Asthma (chronic inflammation of the airway tissues) and other chronic lung disorders. Antihistamines can worsen chronic lung disorders such as asthma, emphysema (condition in which the air sacs in the lungs are damaged or collapse, interfering with the normal exchange of oxygen and carbon dioxide) and chronic bronchitis (a condition in which the breathing passages to and from the lungs have become irritated, decreasing air flow and resulting in greater mucus production). Antihistamines may dry the mucus in the chest and lead to breathing problems. 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). Medications that combine antihistamines with decongestants 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 (an abnormal heartbeat 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 can cause increased enzyme production that can interfere with these conditions.
  
  
• Glaucoma (disease characterized by increased pressure in the eyeball). Antihistamine use can aggravate glaucoma.

Side effects tend to vary according to the type of antihistamine used. For example, older types 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.

A person should immediately contact a physician if they experience any of the following side effects of antihistamines:

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

Less serious side effects of antihistamines include:

• Drowsiness
• Dry mouth or a bitter taste in mouth
• Nausea
• Restlessness or irritability
• Blurred vision
• Constipation

Children or elderly people taking antihistamines may experience more intense or additional side effects such as:

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

Patients should consult their physician 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 treat 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. Medications 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 to treat 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 these drugs with antihistamines can worsen the side effects associated with both drugs.
  
  
• Ketoconazole and itraconazole. Medicine used to treat fungal infections. These drugs cannot be taken with certain antihistamines.
  
  
• 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 physician immediately:

• Unsteadiness, tremor or convulsions of the body
• Flushing or redness of the face
• Rapid heartbeat or low blood pressure

In addition, patients experiencing antihistamine overdose may experience 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 for 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. Though 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.

Breastfeeding mothers should not be overly concerned with the use of antihistamines. However, they should still contact their physician before taking the drug. The medication is passed through breast milk, but it is in such a diluted form (1/100th of the original dose taken) that it should not represent a significant problem. Although antihistamine use can lower the amount of milk produced by a 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 jitters. 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. Convulsions are more likely to occur in children who take antihistamines than in adults. If a child experiences a seizure, 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 medication or dosage.

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 regarding their conditions. Patients may wish to ask their doctor 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. What side effects may I develop?
   
   
5. For what side effects should I seek medical attention?
   
   
6. How long will it take for the drug to take effect?
   
   
7. How long will I have to take this drug?
   
   
8. Are there medications, nutritional supplements or herbal medications I should avoid while taking this drug?
   
   
9. How will I know if the drug is working?
   
   
10. What are other possible treatments if the drug does not work?