Antihistamine usefulness in allergies

Allergies occur when the body’s immune system overreacts to usually non-harmful foreign substances. For example, when you ingest, inhale, or come into contact with something you are allergic to. The body will think it is harmful, hence it will react to try and get rid of it. Allergy symptoms may include sneezing, itching, a runny nose, swelling, nausea, vomiting, and watery eyes. In some cases, allergies can result in life-threatening symptoms such as low blood pressure (anaphylaxis) and swelling of the airway. Common examples of allergens are dust, food, medicine, pollen, and bee stings.

Typically, allergies last for as long as one is exposed to the allergen. Symptoms may be experienced within minutes of exposure, while some may appear after hours and others days later. Avoiding allergens is the ideal measure for allergies. If allergens can’t be avoided there are medications that might be helpful in reducing and managing the symptoms.

Antihistamine

Antihistamines are a class of medicines that are commonly used to treat allergy symptoms. They work by blocking the effect of histamine, a chemical that is produced and released in the body during allergic reactions. Antihistamines come in a number of forms, including capsules, tablets, creams, liquids, and nasal sprays.

Most antihistamines are classified as over-the-counter medication; they can also come in a prescription form. Antihistamines can be found as ingredients in the following over the counter medications: flu and cold products, decongestants, cough suppressants, and pain relievers.

Types of antihistamines

Antihistamines are typically divided into 2 main groups.

1 First generation (sedating) – such as diphenhydramine, chlorphenamine, and promethazine. They typically have more side effects, as they can cross the blood brain barrier into the central nervous system causing drowsiness.

2 Second generation (non-sedating/drowsy) - such as cetirizine, loratadine, and fexofenadine.

Other way in which healthcare professionals group antihistamines is based on which histamine receptor they block:

1 H1 receptors - H1 receptors are found throughout the body, including the brain, blood vessels, and smooth muscle cells in the airways. H1 receptor stimulation results in allergic reactions and anaphylaxis. Thus, H1 receptor antihistamines are mainly for allergy symptom treatment.

2 H2 receptors - H2 receptors are mainly found in smooth muscle cells, heart cells, and cells in the stomach that are responsible for releasing gastric acid. H2 antihistamines treat certain gastrointestinal conditions, such as peptic ulcers, gastritis, and chronic acid reflux. Examples of H2 antihistamines include cimetidine, famotidine, and nizatidine.

Antihistamine side effects

First-generation antihistamine

-          sleepiness (drowsiness), reduced coordination, judgment, and reaction speed. Don’t operate machinery or drive while on these antihistamines

-          dizziness

-          dry mouth and dry eyes

-          blurred or double vision

-          low blood pressure

-          constipation and difficulty urinating

Second-generation antihistamines

-          headache

-          dry mouth

-          cough

-          sore throat

-          abdominal discomfort or pain

H2 antihistamines

-          diarrhea or constipation

-          fatigue

-          muscle and joint pain

-          dizziness

-          headache

-          milky nipple discharge

-          enlarged male breast tissue

When to take antihistamines

Before taking antihistamines, one should seek advice from a pharmacist or doctor.  It is important to know the following: the dose, how to take the antihistamine, how many times a day, and what to do if you miss a dose or take too much. The advice you will receive will vary depending on the exact antihistamine you are taking.

When you take your medication will mostly depend on the type of allergy you have and the severity. For allergic reactions to insect bites, you will take medication only when you have symptoms, while for people who have pet allergy and own pets may need to take antihistamines daily to help control the symptoms or prevent them.  Antihistamines may also be taken in anticipation of contact to allergens.

For most people with environmental allergies, symptoms are often worse during the morning, so taking antihistamines at bedtime may be beneficial.

Antihistamine overdose

An antihistamine overdose is more likely to occur with first-generation H1 antihistamines.  Overdoses happen when you take too much of a drug, in this case antihistamines. One should avoid taking first-generation antihistamines with hypnotics or sedatives as this increases the potential of an overdose. Symptoms of an antihistamine overdose include:

-          hallucinations

-          agitation

-          fast heart rate

-          intense drowsiness

References

All About Allergies. (n.d.). Cleveland Clinic. Retrieved 3 August 2025, from https://my.clevelandclinic.org/health/diseases/8610-allergies

Antihistamines. (n.d.). Cleveland Clinic. Retrieved 3 August 2025, from https://my.clevelandclinic.org/health/treatments/antihistamines

Antihistamines. (2017, October 17). Nhs.Uk. https://www.nhs.uk/medicines/antihistamines/

Australia, H. (2024, October 23). Antihistamines [Text/html]. Healthdirect Australia. https://www.healthdirect.gov.au/antihistamines

What’s the Difference Between First-Generation and Second-Generation Antihistamines? (n.d.). WebMD. Retrieved 3 August 2025, from https://www.webmd.com/allergies/difference-between-first-generation-antihistamines-second-generation-antihistamines

Assessed and Endorsed by the MedReport Medical Review Board