Antihistamines are a class of medications used to counteract the effects of histamine, a chemical released during an allergic reaction that causes sneezing, a runny nose and itching. They are most frequently used to relieve the symptoms associated with allergies.
Antihistamine drugs may also be used to relieve or prevent symptoms associated with colds and the flu. Some experts have disputed the effect of antihistamine medications in effectively treating cold symptoms. However, many others maintain that antihistamines are effective in treating certain cold- or flu-related symptoms, including congestion, itchy eyes, a runny nose and sneezing.
Antihistamines work by blocking the histamines released by body cells. Histamines attach to cells in the body and stimulate them, causing symptoms such as itching, sneezing, a runny nose and watery eyes. Antihistamines have a molecular structure that resembles that of histamine, so they attach to the cell receptors that would otherwise accept histamines. With the effects of the histamines interrupted, the severity of symptoms is reduced.
Antihistamine drugs may likewise interrupt the development or severity of cold and flu symptoms, although this is disputed by some experts. They point out that antihistamines work by blocking the effects of histamines, which are not involved in the symptoms of colds or the flu. They also maintain that even if antihistamines are effective in reducing symptoms of these viral infections, antihistamines are likely to block the flow of mucus from the nose, thus actually trapping the cold or flu virus in the body.
Other experts maintain that antihistamine drugs are an effective way to relieve symptoms such as a runny nose associated with colds and the flu. Many cold and flu medications contain antihistamines. Patients are urged to consult with a physician as to whether or not they should use a cold or flu medication that contains antihistamines.
Types and differences of antihistamines
Some of the antihistamine formulations that are often used to treat colds and the flu include:
- Antihistamine/decongestant combinations. These drugs are used to treat a stuffy nose, runny nose and sneezing. The antihistamines in these drugs prevent the effects of histamines released by the body that trigger certain symptoms. Decongestants narrow the blood vessels, lessening nasal congestion.
- Antihistamine/decongestant/analgesic combination. These drugs provide the same treatment response as antihistamine/decongestant combinations, but also add an analgesic that helps relieve fever, headache, and aches and pains associated with colds and the flu.
- Antihistamine/decongestant/anticholinergic combinations. These drugs provide the same treatment response as antihistamine/decongestant combinations, but also add an anticholinergic (an antispasmodic drug) that produces a drying effect in the nose and chest and helps open the airways.
Patients who wish to purchase antihistamine and decongestant combinations that contain the drug pseudoephedrine will need to speak with a pharmacist, show a form of identification and sign a logbook. In addition, any antihistamine/decongestant combinations that contain the ingredient phenylpropanolamine have been found to be unsafe by the U.S. Food and Drug Administration. Patients with old medications at home that contain this ingredient are encouraged to dispose of those medications.
Antihistamine drugs may also be found in medications formulated to treat coughing, such as antitussives (cough suppressants) or expectorants (which stimulate coughing to expel excess mucus).
Some common antihistamine medications include:
Generic Name Brand Name(s)
Bromphen, Cophene-B, Dimetapp, Dimetane, Nasahist B
Chlo-Amine, Chlorate, Chlor-Trimeton, Deconamine, Gen-Allerate, Phenetron, Telachlor, Teldrin
Aller-Chlor, Contac, Tavist
Calm X, Dramamine, Dramanate, Dinate, Hydrate, TripTone
Banophen, Benadryl, Compoz, Diphenhist, Dormarex 2, Genahist, Hyrexin, Nytol, Siladryl, Sominex, Twilite, Unisom
Atarax, Hyzine-50, Vistaril
Some common antihistamine/decongestant combinations include:
Generic Name Brand Name(s)
Chlorpheniramine and Pseudoephedrine
Allerest Maximum Strength, Chlor-Trimeton, Colfed-A, Deconamine, Kronofed-A Kronocaps, Novafed A, PediaCare Cold Formula, Pseudo-Chlor, Rescon, Tanafed
Chlorpheniramine, Pyrilamine and Phenylephrine
Atrohist Pediatric Suspension Dye Free, R-Tannamine, Triotann
Brompheniramine and Pseudoephedrine
Bromadrine TR, Bromfed, Bromfenex, Iofed, Lodrane, Respahist, ULTRAbrom
Some common antihistamine/decongestant/analgesic combinations include:
Generic Name Brand Name(s)
Chlorpheniramine, Pseudoephedrine and Acetaminophen
Actifed Cold & Sinus, Alka-Seltzer Plus Cold Medicine, TheraFlu/Flu & Cold Medicine
Diphenhydramine, Pseudoephedrine and Acetaminophen
Tylenol Flu NightTime Medication
Dexbrompheniramine, Pseudoephedrine and Acetaminophen
Drixoral Cold & Flu
Some common antihistamine/decongestant/anticholinergic combinations include:
Generic Name Brand Name(s)
Chlorpheniramine, Phenylephrine, and Methscopolamine
AH-chew, D.A. Chewable, Dallergy, Dura-Vent/DA, Extendryl, Mescolor, OMNIhist L.A., Stahist
Antihistamines used to treat colds most often are administered though tablets, liquids or nasal sprays.
Antihistamines can also be divided into the following groups:
- H1 antihistamines. These antihistamine medications are used to block H1 receptors present in many types of tissue (including the small blood vessels known as capillaries), and help to prevent an allergic response. Because H1 receptors are the most common type of receptor, this is the most common and frequently used form of antihistamine.
- Nonsedating antihistamines. Because of the sleepiness experienced by almost half of all patients taking early forms of antihistamines, nonsedating (non-drowsy) forms of the medication were developed. Although early forms of nonsedating antihistamine medications were found to be unsafe, newer types (e.g., loratadine and fexofenadine) are considered both safe and effective. The early, unsafe types of antihistamines have been removed from the market in the United States.
- Lightly sedating antihistamines. Some forms of newer antihistamines are occasionally found to cause sedation in people who use them. For this reason, the FDA has decided that these forms of antihistamines, such as cetirizine, should be called “lightly sedating” instead of “nonsedating.”
Conditions of concern with antihistamine medications
Antihistamines can aggravate certain medical conditions in some patients. Although physicians may prescribe antihistamines to patients with some of these conditions, patients should avoid over-the-counter antihistamines if they have been diagnosed with any of the following conditions:
- Asthma. Antihistamines can provoke an asthma attack in some individuals.
- Ear infection. 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 types of medications.
- Enlarged prostate, intestinal obstruction or urinary tract block. Antihistamine use can aggravate these conditions.
- Heart problems. Antihistamines can produce heart palpitations (strong, fast, irregular or abnormal heartbeats) 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. Antihistamine medications may cause increased enzyme production that can interfere with these conditions.
- Glaucoma. Antihistamine use can aggravate glaucoma, a disease that causes increased pressure in the eye.
- Side effects
Potential side effects
Some types of antihistamines produce slightly different side effects. For example, older forms of antihistamines are known to affect coordination and make it harder to concentrate. More recent versions no longer have this effect.
Side effects associated with antihistamines may diminish after several days of use. However, a person should immediately contact a physician upon experiencing any of the following serious side effects:
- Low blood pressure
- Inability to urinate
- Labored breathing
- Increased heartbeat
Less serious side effects of antihistamines include:
- Drowsiness (more common with over-the-counter antihistamines)
- Dry mouth or a bitter taste in mouth
- Restlessness or irritability
- Blurred vision
Children or elderly persons taking antihistamines may experience more intense side effects or other conditions, such as:
- Confusion and nervousness
- Dryness of the nose, mouth or throat
- Difficult or painful urination
Drug or other interactions
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 certain types of antihistamines in the blood, and may increase the severity of side effects.
- Anticholinergics (antispasmodics). Medicine used to treat airway, abdominal and stomach spasms as well as cramps. Anticholinergics can cause drowsiness (a common side effect also associated with antihistamines) and therefore should not be combined with antihistamines. However, it should be noted that some medications are formulated to contain both antihistamines and anticholinergics. Patients are urged to consult a physician before using these or any other medications.
- 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 these drugs with antihistamines may 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 the antihistamine fexofenadine in the blood. Itraconazole should also be avoided.
- Aspirin. Symptoms associated with the effects of using 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
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:
- How to use
Pregnancy use issues
Pregnant women are encouraged to 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 the antihistamines 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 miscarriage. This category of antihistamines includes fexofenadine and desloratadine.
Breastfeeding mothers are encouraged to contact their physicians before taking antihistamines. The medication is passed through breast milk, although it is in such a diluted form that it should not represent a significant problem. While antihistamine use can lower the amount of milk produced by a new mother, this condition is not permanent.
Child use issues
Some experts maintain that over-the-counter medications that combine antihistamines with decongestants are usually ineffective when used in preschool-age children, particularly those under the age of 12.
There also are several side effects to consider before giving a child an antihistamine. These side effect may vary greatly from the side effects that may occur in adults. Side effects 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. This may be less common in children than in adults.
- Hyperactivity or jitteriness. Some children react to antihistamine medications with varying degrees of restlessness. Concerned parents should contact their child’s pediatrician to determine the proper dosage or medication type.
- 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 pediatrician to determine the proper dosage or medication type for their child.
Parents are encouraged to discuss their concerns about antihistamine use with their child’s pediatrician.
Elderly use issues
Elderly patients are typically more susceptible to the effects of antihistamines. For this reason, older adults taking antihistamines often experience:
- Confusion, nervousness or irritability
- Dryness in the nose, mouth or throat
- Difficult or painful urination
Questions for your doctorPatients 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?