What to Know About Antihistamines and Asthma

Antihistamines can play a crucial role in managing allergic asthma. Allergic asthma, also known as atopic or allergy-induced asthma, is triggered by exposure to an allergen. This exposure sets off an overreaction of the immune system, during which the body releases a chemical called histamine.

Histamine is responsible for allergic symptoms. In people with allergic asthma, histamine causes airway (bronchiole) constriction. It also causes excess mucus production. Together, these effects impede airflow to the lungs.

Antihistamines prevent histamine from binding to receptors that trigger symptoms in both conditions.

Antihistamines are available over the counter (OTC) and prescription as tablets, capsules, liquid gels, eye drops, and nasal sprays. Common 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.

This article explains the considerations for using antihistamines when you have asthma.

Antihistamine Uses

People primarily use antihistamines 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).

To Treat Allergies

These medications are not a first-line treatment for allergic asthma. That's because histamine is not the only chemical involved in asthma symptoms. However, they can help to relieve asthma symptoms triggered by histamine, including:

• Inflammation
• Bronchoconstriction (narrowing of the airways)
• Excess mucus in the airways

When antihistamines are used to control allergy symptoms, there can sometimes be a slight improvement in allergic asthma.

Histamine may play a more significant role in specific subtypes of allergic asthma, but this isn't well understood. Therefore, people use antihistamines for allergy symptoms with a limited expectation for improving allergic asthma.

Combined Therapy for Asthma

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
• Inhaled corticosteroids to prevent symptoms
• Leukotriene modifiers to control mucus production, airway constriction, and inflammation
• Monoclonal antibody medications, such as Xolair (omalizumab)

Antihistamines and leukotriene modifiers are commonly taken as a combination therapy for allergies. However, they may have an enhanced effect in treating mild to moderate persistent asthma.

Establishing a Treatment Plan

Talk to a 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.

Tests

A doctor may want to do a blood or skin test to confirm you have an allergy. That way, you will know which things to avoid, if possible.

They also may want to measure your lung function with spirometry. This test estimates your degree of airway constriction before and after using an inhaler.

Once the tests are complete, your asthma will be classified based on severity. This classification will influence your treatment plan and the combination of medications your doctor prescribes.

Allergy Treatment

The best allergy treatment is prevention. You may 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
• Pests (like cockroaches)

Suppose you can avoid an allergen through mold remediation or pest control. In that case, you may be able to eliminate allergy symptoms without medications, or you may only need to use them for a short time.

If it's not possible to avoid allergens or if your symptoms persist, your healthcare provider may also discuss alternative treatments, including medication or immunotherapy (allergy shots or tablets).

Inhalers

An inhaler is a prescription medication that you breathe directly into the lungs. They are essential for asthma treatment and may be short-acting or long-acting.

A rescue inhaler (Albuterol) is a short-acting medication that you use to relieve symptoms of an allergy attack. In addition to these short-acting medications, your healthcare provider may recommend long-acting controller medications if your asthma symptoms are frequent and severe. These medications include inhaled corticosteroids and leukotriene modifiers.

Inhaled medications for daily use are often unnecessary for people with mild or moderate asthma. So, talk to a doctor to see if your asthma is severe enough to warrant a long-acting medication.

Precautions and Contraindications

Antihistamines are generally considered safe. That's often true even in pregnancy and while breastfeeding. Even so, if you're pregnant or breastfeeding, you should check with your healthcare provider before taking an antihistamine.

General Contraindications

However, some people shouldn't take antihistamines. For example, people with known allergies or hypersensitivities to antihistamines should avoid them.

Also, certain medical conditions can make taking OTC or prescription antihistamines risky. For instance, if you have phenylketonuria (PKU) (the inability to break down the amino acid phenylalanine), be aware that some quick-dissolve antihistamine tablets contain aspartame, which contains phenylalanine.

In addition, you should not take Xyzal if you have end-stage renal disease or are undergoing dialysis. Doctors typically prescribe a lower antihistamine dose to those with any stage of kidney impairment or liver disease. That's because of an increased risk of toxicity.

Anticholinergics Risks

First-generation antihistamines (e.g., Benadryl, Ala-Hist IR) are considered anticholinergic drugs. These types of medications block the activity of acetylcholine, the neurotransmitter that sends messages within the brain and throughout the central nervous system.

Anticholinergics can worsen certain conditions or lead to severe complications. Talk with a healthcare provider before taking one of these drugs, especially if you have any of the following conditions:

• Urinary obstruction
• Benign prostatic hypertrophy (enlarged prostate)
• Glaucoma
• Gastrointestinal obstructive disorders
• Hyperthyroidism
• Heart problems
• Hypertension (high blood pressure)
• Epilepsy

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

Dosage

Antihistamine dosage depends on the medication and may vary depending on your overall treatment plan. For example, some people with allergies may take an antihistamine daily year-round, while others may only take it seasonally or on an as-needed basis.

OTC antihistamines commonly come in tablet, chewable, dissolvable, and syrup preparations. Prescription medications, like Clarinex and Xyzal, typically come in tablets or syrup.

Modifications

A healthcare provider may recommend a lower starting dose of antihistamines if you have liver or kidney impairment. That's because you may not clear antihistamines as efficiently, increasing the risk of toxicity.

Older adults are more likely to have impaired liver or kidney function and may benefit from lowered dosages.

If you have kidney or liver disease, talk to a healthcare provider before starting antihistamines to determine a correct starting dose.

How to Take and Store

You can take most antihistamines with or without food.

People generally take second-generation antihistamines in the morning. However, suppose your healthcare provider prescribes both an antihistamine and a leukotriene modifier for allergic asthma. In that case, it's common to take the antihistamine in the morning and the leukotriene modifier in the evening.

You should store most antihistamines at room temperature. Read the product label for exact ranges to ensure they don't get too warm or cold.

Side Effects

Antihistamines are typically well-tolerated. However, they do carry the risk of side effects, especially first-generation antihistamines. Side effects are also more likely when you take high doses of antihistamines.

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 amplify this antihistamine side effect.

If you are older than 60, you are at greater risk of becoming drowsy with an antihistamine and may have an increased risk of falling.

Tell your healthcare provider if you feel dizzy after taking an antihistamine. Your medication may need to be adjusted or changed if you have allergic asthma.

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

If you take any other medications, ask a doctor to check for any interactions that antihistamines may have before taking them. Of course, it's always worth consulting with your pharmacist about this as well.

Never assume that your doctor or pharmacist knows about the medications you take. If you take any of the following medications, talk to a healthcare provider before taking antihistamines:

• Antibiotics and antifungals
• Asthma medications
• Muscle relaxants
• Pain medications
• Psychiatric medications
• Sedatives

Research has found that central nervous system (CNS) depressants may interfere with antihistamines. Specifically, when combined with alcohol and other sedatives, hypnotics (sleeping pills), pain medications, or tranquilizers, antihistamines can reduce alertness and become dangerous. Therefore, it's best not to combine these medications in most cases.

In addition, older adults may have an increased risk of falls and hospitalization if they combine muscle relaxants with antihistamines.

Summary

Antihistamines are not a first-line treatment for asthma. However, they can help reduce allergy symptoms, which can sometimes help manage allergic asthma. Antihistamines are not suitable for everyone. Some medications and health conditions may contraindicate antihistamine use. Antihistamines are available OTC, but be sure to discuss taking them with a healthcare provider or pharmacist first, especially if you already take medication to manage asthma or have health conditions.

A Word From Verywell

While studies suggest histamines may play a critical role in developing specific subtypes of allergic asthma, more research is needed to explore this link and whether or not antihistamines can be an effective treatment.

If your doctor has 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.