What to Know About Antihistamines and Asthma

Drugs for allergy symptoms

Antihistamines can play a key role in managing allergic asthma. 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 in which a chemical called histamine is released.

Histamine is responsible for symptoms experienced by people who have allergies to common environmental triggers as well as those who have allergic asthma—around 60% of all asthmatics. In people with allergic asthma, histamine causes constriction of the airways (bronchioles) and production of excess of mucus, which together impede the flow of air to the lungs. Antihistamines prevent histamine from binding to receptors that trigger symptoms in both conditions.

Antihistamines are available over the counter and by prescription as tablets, capsules, liquid gels, eye drops, and nasal sprays. 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
GIPhotoStock / Getty Images

Uses

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

These medications are not a first-line treatment for allergic asthma, but they can help to relieve the asthma symptoms triggered by the release of histamine: inflammation, bronchoconstriction (narrowing of the airways), and production of excess amounts of mucus in the airways.

Antihistamines are not considered a primary treatment for allergic asthma, however, as histamine is not the only chemical involved in the inflammation that characterizes the disease. However, when used to control allergy symptoms, there can sometimes be a slight improvement in allergic asthma.

Histamine may play a more significant 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.

If you have allergic asthma, your healthcare provider may recommend antihistamines in combination with other asthma medications, among them:

  • Short-acting beta-agonists (SABAs), also known as rescue inhalers, that are breathed in using an inhaler to quickly ease acute asthma symptoms
  • Inhaled corticosteroids, which are used on a regular basis (often daily) to prevent symptoms
  • Leukotriene modifiers, an alternative to inhaled corticosteroids that target molecules that contribute to mucus production, airway constriction, and inflammation
  • Monoclonal antibody medications, such as Xolair (omalizumab), an injectable that is sometimes prescribed when other medications don't control asthma symptoms well enough

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

Talk to your healthcare provider about whether an antihistamine should have a place in your asthma management plan. They'll want to consider the specific symptoms you experience and how often they occur.

They also may want to do a blood or skin test to confirm you have an allergy. If you do, you may be able to prevent symptoms by steering clear of the allergen involved, but this can be challenging: Among the most common allergens are things that often are hard to avoid, such as pollen (from trees, grasses, weeds), mold, dust mites, cats and dogs, and pests (cockroaches).

If it's possible for you to avoid an allergen, through mold remediation or pest control, you may be able to eliminate allergy symptoms without medications, or you may only need to use them for a short time. Your healthcare provider 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 healthcare provider about all medications, supplements, and vitamins that you currently take. While some drugs pose minor interaction risks, others may 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 how often you have asthma symptoms and how severe they are, your healthcare provider may recommend one or more long-term controller medications, such as inhaled corticosteroids and/or leukotriene modifiers.

Note that although inhaled medications typically 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 asthma. If you use an inhaler daily to manage asthma, talk to your healthcare provider about how the new guidelines might affect your treatment.

Precautions and Contraindications

Certain medical conditions can make taking OTC or prescription antihistamines risky or even too unsafe:

  • Allergy or hypersensitivity to antihistamines
  • Pregnancy: Research suggests most types of antihistamines are safe to take during pregnancy. Even so, if you're pregnant you should check with your healthcare provider before taking an antihistamine or alert them if you become pregnant while taking an antihistamine.
  • Nursing: Antihistamines are generally regarded as safe for people who are breastfeeding.
  • 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 healthcare provider before taking a combination product.

First-generation antihistamines (e.g., Benadryl, Ala-Hist IR) are considered anticholinergic drugs since they block activity of acetylcholine, 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 healthcare provider 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 come in tablets or as an oral solution.

Antihistamine Generation Availability Typical Starting Dose
Benadryl 1st OTC Adults and children 12 and up: 25 to 50 mg taken every four to six hours as needed (but no more than six doses in 24 hours) Children 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 6 and up: 10 mg once a day 
Zyrtec 2nd  OTC  Adults and children 6 and up: 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 and up: 5 mg once a day Children ages 6 to 11: 2.5 mg a day Children 1 to 5: 1.25 mg (1/2 tsp of oral solution) once a day Children 6 months to 11 months: 1 mg once a day
Xyzal 2nd Prescription Adults and children 12 and up: 5 mg taken once a day in the evening Children 6 to 11: 2.5 mg (1/2 tablet or 1 tsp oral solution) taken once a day in the evening.

Modifications

Your healthcare provider 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 healthcare provider 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, with 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. Side effects are also more likely when antihistamines are 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 healthcare provider 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
  • Difficulty urinating
  • Yellowing of skin
  • Weakness

Warnings and Interactions

While your healthcare provider should check for any interactions that 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 increase 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 if these medications 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. 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 healthcare provider. There are plenty of other treatment options that you can try.

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