What to Know About Antihistamines and Asthma

Drugs for allergy symptoms

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Antihistamines can play a key role in treating allergic asthma—the most common type of asthma, affecting about 60% of people with the condition.Also known as atopic or allergy-induced asthma, allergic asthma is triggered by exposure to an allergen that in turn sets off an overreaction of the immune system. When this occurs, a chemical called histamine is released, which causes bronchoconstriction—narrowing of the bronchioles (airways) of the lungs—and a build-up of mucus in those airways. Antihistamines do exactly what the name suggests—block the release of histamine and its effects on the airways.

Antihistamines are available over the counter and by prescription in tablet, capsule, liquid gel, eye drop, and nasal spray forms. Common over-the-counter (OTC) antihistamine brands include Allegra (fexofenadine), Benadryl (diphenhydramine), Claritin (loratadine), Xyzal (levocetirizine), and Zyrtec (cetirizine). Clarinex (desloratadine) is a prescription brand. There are also generic forms of some antihistamines.

Pink 25mg Diphenhydramine antihistamine pills from medicine bottle
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Uses

Antihistamines primarily are used to treat or prevent nasal allergy symptoms including sneezing, runny nose, itchy or watery eyes, and itchy nose or throat. They're effective for seasonal and year-round allergies, as well as other causes of itching and urticaria (hives).

Antihistamines are not considered a primary treatment for allergic asthma, however, as this condition involves mediators other than histamine that cause inflammation and bronchoconstriction. However, when they are used to control allergy symptoms, there can sometimes be a slight improvement in allergic asthma.

Histamine may play more of a role in certain subtypes of allergic asthma, but this isn't well understood. Therefore antihistamines are mainly given for allergy symptoms with a limited expectation for improving allergic asthma.


Antihistamines are not effective for acute asthma symptoms and should never be taken for an asthma attack.

Your doctor may recommend antihistamines in combination with one or more asthma medications if you have allergic asthma. These include:

  • Inhalers: 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.
  • Leukotriene modifiers: An effective alternative option to inhaled corticosteroids, these drugs target molecules that contribute to mucus and airway constriction and inflammation in asthma.
  • Monoclonal antibody medications, such as Xolair (omalizumab), target and block immunoglobulin E (IgE) antibodies that are responsible for an allergic response.

Antihistamines and leukotriene modifiers—e.g., Accolate (zafirlukast), Singulair (montelukast), and Zyflo (zileuton)—are commonly taken as a combination therapy for allergies, but they may have an enhanced effect in the treatment of mild to moderate persistent asthma.

Before Taking

Your doctor will ask you about your symptoms and their frequency, and they may want to do a blood or skin test to confirm an allergy. Common allergens include pollen (from trees, grasses, weeds), mold, dust mites, pets (cats, dogs), and pests (cockroaches).

If it's possible for you to avoid an allergen, such as through mold remediation or pest control, you may be able to eliminate allergy symptoms without the use of medications, or only need to use them for a short time. Your doctor may also discuss alternative treatments, such as immunotherapy (allergy shots or tablets).

They also may want to measure your lung function—for example, with spirometry—to estimate your degree of airway constriction before and after using an inhaler. Your asthma will also be classified based on severity, which will influence your treatment plan 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.

A rescue inhaler is important if you have asthma. Depending on your asthma frequency and severity, your doctor may next recommend one or more long-term controller medications, such as inhaled corticosteroids and/or leukotriene modifiers. Note that while typically such inhaled medications have been prescribed to be used daily, according to updated recommendations for asthma management by the National Institutes of Health issued in December 2020, this no longer is regarded as necessary for those with mild to moderate persistent. If you use an inhaler daily to manage asthma, talk to your doctor about how the new guidelines might affect your treatment.

To address allergy symptoms, such as a runny nose, sneezing, and itchy, watery eyes, an OTC antihistamine medication may be taken first. If you are not able to get these allergy symptoms under control with an OTC antihistamine, your doctor may recommend prescription antihistamines in addition to your asthma medications.

Precautions and Contraindications

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

  • 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.
  • Pregnancy: If you are pregnant, discuss treatment options with your doctor. Research suggests that taking most types of antihistamines during pregnancy is not linked to birth defects, but additional studies are needed. Of particular note, there are no adequate or controlled human studies for Clarinex, which animal studies suggest may be harmful to fetuses. As such, it is not typically given during pregnancy.
  • Nursing: Many antihistamines can be transferred to an infant in breastmilk and so are typically not recommended in this situation.
  • 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 antihistamine tablets contain aspartame, which is dangerous for people 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 before taking a combination product.

First-generation antihistamines (e.g., Benadryl, Ala-Hist IR) are considered anticholinergic drugs since they block activity of cetylcholine, the neurotransmitter that transmits messages within the brain and throughout the central nervous system.

Anticholinergics can worsen certain conditions or lead to serious complications. Talk with your doctor before taking one of these drugs if you have any of the following conditions that may prohibit their use:

Research has also found a link between anticholinergics and an increased risk of dementia and Alzheimer's disease. As none of the second-generation antihistamines are anticholinergics, those with a family history of Alzheimers—or those taking Benadryl frequently—may want to switch to one of these drugs.

Dosage

The dosage of antihistamine depends on the medication and may vary depending on your overall treatment regimen and any combination therapies. Some people with allergies may take an antihistamine daily year-round, while others may only take it seasonally or on an as-needed basis.

Claritin and Zyrtec come as tablets or a 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.

Modifications

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, increasing the 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

Most antihistamines can be taken with or without food.

Second-generation antihistamines are typically taken in the morning. If your doctor prescribes both an antihistamine and a leukotriene modifier for allergic asthma, it's common to take the antihistamine in the morning and the leukotriene modifier in the evening.

Storage recommendations vary 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. (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 typically are well tolerated but do carry the risk of side effects, especially first-generation antihistamines and when taken at high doses.

Common

Common side effects include:

  • Drowsiness
  • Dizziness
  • Dry mouth
  • Hoarseness
  • Nausea

Do not drive or engage in activities that require alertness when you first take an antihistamine until you know how it affects you. Asthma medications, especially rescue inhalers, may also cause dizziness and can amplify this antihistamine side effect.

Let your physician know if you feel dizzy after taking an antihistamine. Your medication may need to be adjusted or changed if you have allergic asthma.

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 with first-generation antihistamines than second-generation antihistamines for people of all ages.

Severe

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

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

Warnings and Interactions

While your doctor should check for any interactions antihistamines may have with other drugs you are taking, it's always worth consulting with your pharmacist about this as well.

Of particular note for those being treated for asthma:

  • 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 levels of antihistamines in the blood. Mixing Clarinex with azithromycin can also increase levels of Clarinex.

Other drugs that can interact with antihistamines include:

  • 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, it's best not to combine these medications.
  • Prozac (fluoxetine): This selective serotonin reuptake inhibitor (SSRI) 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: Older adults may have an increased risk of falls and hospitalization with these medications and if they are taken in combination with antihistamines.

A Word From Verywell

While studies suggest histamines may play a critical role in the development of certain subtypes of allergic asthma, more research is needed to explore this link and whether or not antihistamines can be an effective treatment for all. If you have been prescribed an antihistamine and it isn't adequately controlling your allergy symptoms and allergic asthma (or it stops working over time), speak with your doctor. There are plenty of other treatment options that you can try.

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