What Does An Antihistamine Do? MOA And Uses

Last Updated: June 10, 2020

Authored by Sharon Levy, MD, MPH

Almost everyone can remember using an antihistamine for a runny nose at some point in their life. This is a class of medications that is used to obtain relief from symptoms of allergic reactions. People frequently use an antihistamine for hives, itchy skin, skin rash, bee stings, and bug bites. Other common uses of these drugs are antihistamine for cold and flu symptoms such as sneezing, itchy throat, and red and watery eyes. What are some of the lesser-known uses of these medicines? What is their mechanism of action?

How Does an Antihistamine Work?

The human body releases a chemical called histamine in response to injury, inflammation, and allergic reactions. Histamine causes several effects, including contraction of smooth muscles, dilation of blood vessels, and increased secretion of acid in the stomach. Antihistamine, as the name suggests, is a class of drugs that tames the effects of the chemical histamine. How does antihistamine work?
Anti-allergy drugs work by blocking the release of histamine from histamine-1 receptors. These receptors are located in the blood vessels, airways, nasal mucosa, and gastrointestinal tract.

Antihistamine for itching and rash works by blocking histamine from binding with the H1 receptors and stimulating them.
Histamine also stimulates receptors in the central nervous system (brain and spinal cord) to make a person more awake and alert. An antihistamine for sleep has the opposite effect and causes drowsiness and sedation. For this reason, alcohol and antihistamine can be a dangerous combination because alcohol is also a central nervous system depressant.
Among the antihistamine classes, first-generation drugs such as diphenhydramine (Benadryl) act on the brain (in addition to the rest of the body). They are therefore more likely to cause side effects such as sedation and dizziness. Due to this mechanism of action, they are the best antihistamine for sleep and reducing anxiety. On the other hand, second-generation products such as fexofenadine (Allegra) and cetirizine (Zyrtec) are less likely to cause drowsiness. This mechanism of action makes them appropriate for use as an antihistamine for itching, hay fever, and skin rash.

Antihistamine Uses

This class of drugs is used to treat a variety of medical conditions and symptoms. Here are some of the uses of these drugs, their benefits, and effectiveness, as well as the potential side effects.

Antihistamine for the allergic reaction

They are very effective in treating symptoms of minor allergies. By their mechanism of action on H1 receptors, they prevent symptoms of allergy such as hives, skin rash, itching, sneezing, and watery eyes. However, it is essential to remember that while an over-the-counter medication may be the best antihistamine for hives, it cannot treat severe allergic reactions such as anaphylaxis because it does not work fast enough.
man with allergic sneezing

Antihistamine for anxiety

Hydroxyzine (Vistaril) is the most commonly used drug from this class for the treatment of anxiety. Due to its mechanism of action on histamine receptors in the brain, it is effective when used in the short-term (for a few weeks). It produces a calming effect and reduces anxiousness. However, it can cause drowsiness and impair concentration. Other unwanted effects may include dry mouth, dizziness, headache, and blurred vision. Even though it is effective and well-tolerated, hydroxyzine is not the first line of treatment for generalized anxiety disorder.

Antihistamine sleep aid

Because first-generation drugs in this class cause drowsiness, they are useful as sleep aids. They are present in many sleep aids available over-the-counter, such as Nytol (diphenhydramine).

For the longer term, natural antihistamine foods may be a safer option, although their efficacy cannot be guaranteed.

Antihistamine for bug bites

It is effective in reducing the immediate and early symptoms of insect bites in both children and adults. In combination with cold compresses, cream or gel formulation of an antihistamine for mosquito bites applied locally can help reduce itching, redness, and swelling. This symptomatic treatment prevents scratching and the risk of secondary bacterial infection of the bite area.

Antihistamine for cough and cold

This offers relief from many of the symptoms associated with flu, including runny nose, sore throat, and itchy and watery eyes. This is an effective symptomatic treatment. Due to their sedative effects, first-generation drugs can help inhibit cough and allergic symptoms by making the person sleepy. However, a doctor or pharmacist must be consulted to identify a suitable antihistamine for infants and children due to this very effect.
woman with cough and cold

Antihistamine for vertigo

Meclizine (Antivert) may be prescribed to people suffering from dizziness, nausea, vomiting, and other symptoms of motion sickness. It is also useful for vertigo caused by inner ear problems. It is used one hour before starting the motion-sickness-inducing activity and taken by mouth in tablet form. Although antihistamine allergy is a rare complication, people using these medications should be vigilant for potentially dangerous symptoms such as severe dizziness, difficulty breathing, and swelling of the face, tongue, or throat.

Antihistamine for nausea

It is believed that the mechanism of action of these drugs on the histamine receptors in the brain makes them useful in the treatment of nausea. Dramamine (dimenhydrinate), Bonine for Kids (cyclizine), and Antivert (meclizine) can be used to prevent motion sickness before it occurs. Phenergan (promethazine) is useful for nausea and vomiting but is more sedating than the others. Some of the non-sedating drugs in this class, such as cetirizine, are not effective for preventing or treating motion sickness because they do not cross the blood-brain barrier.

Antihistamine for asthma

Antihistamines are effective for asthma symptoms because of their mechanism of action, i.e., widening of the airways (bronchodilation). Some of the drugs that are useful for this purpose include cetirizine, fexofenadine, and azelastine.

Antihistamine for post nasal drip

Second-generation drugs such as Claritin are effective in getting rid of post nasal drip after several days of use. These non-drowsy medications work alongside decongestants and saline nasal sprays to provide relief.

Using Antihistamines Safely

This class of medicines is available in many different forms (tablets, liquids, creams, nasal sprays, and eye drops). They are used to effectively treat several medical conditions, including allergies, vertigo, nausea, anxiety, and insomnia. For example, an antihistamine for food allergy can reduce symptoms such as itching, hives, and skin redness. This is achieved through the mechanism of action on histamine receptors distributed throughout the body.
The medications are available both over-the-counter and by prescription for adults and children. Safe use of these drugs requires using them as directed at the recommended dose. Doubling up the dose, taking a dose more often than suggested, or combining two different prescriptions in the class can prove dangerous. Special care is needed when using an antihistamine for babies because of the sedating properties. The use of antihistamine during pregnancy also warrants careful consideration.
Patients should also be aware that antihistamines can cause dependence and even addiction in some cases. Reach out for help if noticed that someone is unable to function normally without allergy medications. Addiction treatment may be required for such patients. Rehabilitation centers mostly offer such services on an outpatient basis.

Page Sources

  1. Wilson AM. The role of antihistamines in asthma management. Treat Respir Med. 2006;5(3):149-58. https://www.ncbi.nlm.nih.gov/pubmed/16696585
  2. Katselou M, Papoutsis I, Nikolaou P, Spiliopoulou C, Athanaselis S.Bioanalysis of antihistamines for clinical or forensic purposes. Biomed Chromatogr. 2017. https://www.ncbi.nlm.nih.gov/pubmed/27037512

Published on: July 31st, 2019

Updated on: June 10th, 2020

About Author

Sharon Levy, MD, MPH

After successful graduation from Boston University, MA, Sharon gained a Master’s degree in Public Health. Since then, Sharon devoted herself entirely to the medical niche. Sharon Levy is also a certified addiction recovery coach.


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