What to Know About Antihistamines and Asthma

Drugs for allergy symptoms

In This Article

Antihistamines are not asthma medications per se, but they are allergy medications that are sometimes taken by those with allergic (or atopic) asthma.

Allergic asthma (also called allergy-induced asthma), meaning it is triggered by an allergen, is the most common type of asthma and affects about 60 percent of those with asthma.

As its name suggests, antihistamines target histamine, a chemical messenger that helps direct your body's response to foreign invaders and sends messages between cells. In allergies, histamine gets released as an overreaction by your immune system to harmless particles that it perceives as foreign, such as pollen or pet dander, and the allergic response begins.

Histamines plays a primary role in nasal allergy symptoms, such as mucus and itching. They can also increase bronchoconstriction (narrowing of airways) and mucus in the airways that can contribute to allergic asthma.

Antihistamines are available over-the-counter and by prescription and come in tablets, capsules, liquid gels, eye drops, or nasal sprays. Common over-the-counter (OTC) antihistamine brands include Zyrtec (cetirizine), Allegra (fexofenadine), Claritin (loratadine), Benadryl (diphenhydramine). Prescription brands include Clarinex (desloratadine) and Xyzal (levocetirizine). There are also generic forms of some antihistamines.

Pink 25mg Diphenhydramine antihistamine pills from medicine bottle
GIPhotoStock / Getty Images


Antihistamines are used to treat or prevent allergy symptoms, such as sneezing, runny nose, itchy or watery eyes, and itchy nose or throat. They can be used for seasonal or year-round allergies and are also used to treat other forms of itching and urticaria (hives).

Due to additional mediators of inflammation and airway constriction in asthma, antihistamines are not considered a direct treatment for asthma, but when they are used to control allergy symptoms, there can sometimes be a slight improvement in allergic asthma.

Studies suggests that histamines may play a critical role in the development of certain subtypes of allergic asthma, but more research is needed to explore this link and whether or not antihistamines can be an effective treatment for these types.

Quick-acting, short-term medications for asthma, such as short-acting beta-agonists (SABAs), known as rescue inhalers, and long-term use of inhaled corticosteroids are the primary treatments for asthma. An effective alternate option for those who will not take or cannot tolerate inhaled corticosteroids are leuokotriene modifiers, which targets molecules called leukotrienes that contribute to mucus and airway constriction and inflammation in asthma. Common leukotreine modifiers for allergies and asthma include Singulair (montelukast), Accolate (zafirlukast), and Zyflo (zileuton). There are also monoclonal antibody medications, such as Xolair (omalizumab), that target and block immunoglobulin E (IgE) antibodies in the bloodstream that are responsible for an allergic response.

Your doctor may recommend antihistamines in combination with one or more asthma medications to manage your allergies and allergic asthma. Antihistamines and leukotriene modifiers are commonly taken as a combination therapy for allergies and may have an enhanced effect in treatment of mild to moderate persistent asthma. If the asthma is severe, your doctor may recommend inhaled corticosteroids.

Off-Label Uses

Benadryl, a first generation antihistamine with more side effects than the newer second-generation options, is sometimes used as an off-label treatment for insomnia since a common side effect is drowsiness. It's also used off-label as a local anesthetic for people who are allergic to other common local anesthetics and as a treatment for oral mucositis, a swelling of the mouth.

Before Taking

Your doctor will ask you about your symptoms and the timing of symptoms and may want to do a blood or skin test to confirm your allergy. Common allergens include pollen (from trees, grasses, weeds), mold, dust mites, pets (cats, dogs), or pests (cockroaches).  If it's possible for you to avoid the allergen, such as through mold remediation or pest control, you may be able to eliminate symptoms without the use of antihistamines or only need to use them for a short time. Your doctor may also discuss alternate treatments, such as immunotherapy (allergy shots or tablets).

For asthma, your doctor may also want to measure your lung function such as through a spirometry device that you exhale into and it gives an estimation of the narrowing of your bronchial tubes. You may be asked to do this before and after using an inhaler. Your asthma will also be classified based on severity, which will influence your treatments and the combination of medications that you are given.

Talk to your doctor about all medications, supplements, and vitamins that you currently take. While some drugs pose minor interaction risks, others may outright contraindicate use or prompt careful consideration as to whether the pros of treatment outweigh the cons in your case.

When you experience a runny nose, sneezing, and itchy, watery eyes from allergies, you may want to try an OTC antihistamine medication first. If you are not able to get your allergy symptoms under control with an OTC antihistamine, your doctor may recommend prescription antihistamines.

Precautions and Contraindications

Certain medical conditions can make taking OTC or prescription antihistamines risky or even prohibit their use.

  • Pregnancy: There are no adequate or controlled human studies for Clarinex and animal studies suggest it might carry fetal risks, so it is not typically advised to take it while pregnant.
  • Nursing: Many antihistamines can be transferred to the infant in breastmilk and are typically not recommended while breastfeeding.
  • Allergy or hypersensitivity to antihistamines: Do not take an antihistamine medication if you have a known allergy or hypersensitivity to it or other types of antihistamines.
  • End-stage kidney failure or dialysis: Do not take Xyzal if you have end-stage renal disease or are undergoing dialysis. Those with mild, moderate, or severe kidney impairment or liver disease are typically given lower starting doses of antihistamines due to an increased risk of toxicity.
  • Phenylketonuria (PKU): Some quick-dissolve tablets of antihistamines contain aspartame that is dangerous for those with PKU.

Warning: Combination Allergy Medications

Antihistamines are also available in combination allergy products with decongestants, such as pseudoephedrine, that can interact with several medications and raise the risk of side effects. Consult your doctor if you plan to take a combination product, especially if you are taking any other drugs.

First-generation antihistamines, such as Benadryl, are considered anticholinergic drugs since they block activity of the neurotransmitter acetylcholine, which transmits messages within the brain and throughout the central nervous system.

Anticholergenics can worsen certain conditions or lead to serious complications. Talk with your doctor before taking Benadryl if you have any of the following conditions that may prohibit the use of anticholinergenics:

Research has also found a link between anticholergenics and an increased risk of dementia and Alzheimer's disease. Those with a family history of Alzheimers or those taking Benadryl frequently, may want to switch to second-generation options that are not considered anticholergenics.


The dosage of antihistamine depends on the medication and may vary depending on your overall treatment regimen and any combination therapies.

Claritin and Zyrtec come tablets or syrup. Allegra comes in 30, 60, and 180 milligram (mg) tablets. Benadryl comes in tablets, capsules, or an oral solution.

Prescription medications Clarinex and Xyzal comes in tablets or an oral solution.

Antihistamine Generation Availability Typical Starting Dose
Benadryl 1st OTC Adults and children ages 12+: 25 to 50 mg taken every four to six hours as needed (but no more than six doses in 24 hours)

Children ages 6 to 11: 12.5 to 25 mg (5 to 10 mL) every four to six hours as needed (but no more than six doses in 24 hours)
Claritin 2nd OTC  Adults and children ages 6+: 10 mg once a day 
Zyrtec 2nd  OTC  Adults and children ages 6+: 5 or 10 mg taken daily
Allegra 2nd OTC Adults and children ages 12+: 120 mg (60 mg twice a day) or 180 mg once a day

Children ages 6 to 11: 60 mg (30 mg twice a day) 
Clarinex 2nd  Prescription Adults and children 12+: 5 mg once a day

Children ages 6 to 11: 2.5 mg once a day

Children 1 to 5 years: 1.25 mg (1/2 tsp of oral solution) once a day

Children 6 to 11 months: 1 mg once a day
Xyzal 2nd Prescription Adults and children 12+: 5 mg taken once a day in the evening

Children ages 6 to 11: 2.5 mg (1/2 tablet or 1 tsp oral solution) taken once a day in the evening.


Your doctor may recommend a lower starting dose of antihistamines if you have liver or kidney impairment since you may not clear antihistamines as efficiently, which puts you at a greater risk of toxicity. For example, the starting dosages may be adjusted to 10 mg every other day for Claritin, 5 mg every other day for Clarinex, 5 mg daily for Zyrtec, and 30 to 60 mg once daily for Allegra.

Elderly patients are more likely to have impaired liver or kidney function and may need to be evaluated and started on these lowered dosages so that they can take the lowest effective amount.

How to Take and Store

Antihistamines can typically be taken with or without food and storage varies by drug. Both Zyrtec and Xyzal should be stored at room temperature that's ideally 68 to 77 degrees F and can be taken on excursions in temperatures ranging from 59 to 86 degrees F. As an alternate option, Zyrtec may also be stored in the refrigerator.

Claritin should be stored in a cool, dry place that's ideally between 36 and 77 degrees F. Clarinex should be kept at 77 degrees F with excursions that can range from from 59 to 86 degrees and should be protected from excessive heat or light. Benadryl should be stored at room temperature that's ideally 68 to 77 degrees F.

Side Effects

Antihistamines are typically well tolerated but do carry the risk side effects, especially amount first-generation antihistamines and those taken at high doses.


Common side effects include:

  • Drowsiness
  • Dizziness
  • Dry mouth
  • Hoarseness
  • Nausea

If you are older than 60, you are at greater risk of becoming drowsy with an antihistamine, and may also have an increased risk of falling. Sleepiness is also more pronounced and likely in first-generation antihistamines, such as Benadryl, than second-generation antihistamines.


If you experience any of the following side effects while taking an antihistamine seek urgent medical attention:

  • Changes in vision
  • Extreme nervousness
  • Racing heartbeat
  • Stomach pain
  • Stop or have difficulty urinating
  • Yellowing of skin
  • Weakness

Warnings and Interactions

Because antihistamines can also react with other drugs, it is important for every doctor that treats you knows your medications. Make sure you bring the pill bottles with you or at least an up-to-date list. If you get a new prescription make sure to ask your pharmacist to check to make sure there will be no dangerous interactions.

Drug interactions may include:

  • Elixophyllin or Uniphyl (theophylline): This medication used to treat asthma, emphysema, and other lung conditions may slightly decrease clearance of Zyrtec from the body.
  • Antibiotics or antifungals: Combining Allegra or Clarinex with ketoconazole or erythromycin can increase blood levels of the antihistamines. Mixing Clarinex with azithromycin can also increase levels of Clarinex.
  • Central nervous system (CNS) depressants: Antihistamines have additive side effects with alcohol and other sedatives, hypnotics (sleeping pills), pain medications, or tranquilizers that can reduce alertness and become dangerous. In most cases, you will want to avoid combining these medications.
  • Selective serotonin reuptake inhibitors (SSRIs): Prozac (fluoxetine) may slightly increase blood concentrations of Clarinex
  • Tagamet (Cimetidine): This drug for gastroesophageal reflux disease (GERD) also contains an antihistamine and may slightly increase blood levels of Clarinex.
  • Muscle relaxants: Elderly individuals may have an increased risk of falls and hospitalizations with these medications and if they are taken in combination. 

Avoid driving or doing activities that require alertness when you first take an antihistamine until you know how it affects you.

If you are pregnant, discuss options with your doctor. Research suggests that taking most types of antihistamines during pregnancy is not linked to birth defects, but additional studies are still needed. 

A Word from Verywell

Antihistamines don't directly treat asthma, so if they don't get your allergy symptoms and allergic asthma under control or stop working for your symptoms over time, there are plenty of other treatment options that you can try. Your doctor may consider adding a nasal steroid, leukotriene modifier, Xolair, or immunotherapy.

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