Antihistamines for allergies
Allergic rhinitis - antihistamine; Hives - antihistamine; Allergic conjunctivitis - antihistamine; Urticaria - antihistamine; Dermatitis - antihistamine; Eczema - antihistamine
Allergens, like pollen, are nothing more than foreign plant antigens. When they get released into the air, you can see and hear the result. When allergens first encounter nasal tissue, sneezing is triggered. This is part of the body’s immune defense. Pollen allergens then encounter the plasma cells in the nose, which respond by producing antibodies. These antibodies attach to mast cells, which are white blood cells containing the chemical histamine. As more antibodies are produced, they cause the mast cells to release histamine, which produces allergy symptoms such as a stuffy and runny nose, sneezing, and watery eyes. These help to remove the invading pollen. Medications called antihistamines can be used to help relieve severe allergy symptoms.
The immune system normally responds to harmful substances such as bacteria, viruses and toxins by producing symptoms such as runny nose and congestion, post-nasal drip and sore throat, and itchy ears and eyes. An allergic reaction can produce the same symptoms in response to substances that are generally harmless, like dust, dander or pollen. The sensitized immune system produces antibodies to these allergens, which cause chemicals called histamines to be released into the bloodstream, causing itching, swelling of affected tissues, mucus production, hives, rashes, and other symptoms. Symptoms vary in severity from person to person.
Mast cells release histamine when an allergen is encountered. The histamine response can produce sneezing, itching, hives and watery eyes.
What are Antihistamines?
Antihistamines are medicines that treat allergy symptoms by blocking the effects of histamine. Antihistamines come as pills, chewable tablets, capsules, liquids, nasal sprays, and eye drops. There are also injectable forms used mainly in health care settings.
How Antihistamines Help
Antihistamines treat these allergy symptoms:
- Congestion, runny nose, sneezing, or itching
- Swelling of the nasal passages
- Hives and other skin rashes
- Itchy, runny eyes
Treating symptoms can help you or your child to feel better during the day and sleep better at night.
How to Take Antihistamines
Depending on your symptoms, you can take antihistamines:
- Every day, to help keep daily symptoms under control
- Only when you have symptoms
- Before being exposed to things that often cause your allergy symptoms, such as a pet or certain plants
For many people with allergies, symptoms are the worst around 4 a.m. to 6 a.m. Taking an antihistamine at bedtime may help you or your child feel better in the morning during allergy season.
What Antihistamine is Right for you?
You can buy many different brands and forms of antihistamines without a prescription.
- Some work for only 4 to 6 hours, while others last for 12 to 24 hours.
- Some are combined with a decongestant, a medicine that dries up your nasal passages.
Ask your health care provider what type of antihistamine and what exact dosage is right for you or your child. Make sure you understand how much to use and how many times a day to use it. Be sure to read the label carefully. Or ask your pharmacist or provider if you have questions.
- Some antihistamines cause less sleepiness than others. These include cetirizine (Zyrtec), desloratadine (Clarinex), fexofenadine (Allegra), levocetirizine (Xyzal), and loratadine (Claritin).
- Do not drink alcohol when you are taking antihistamines.
Also, remember:
- Store antihistamines at room temperature, away from heat, direct light, and moisture.
- Do not freeze antihistamines.
- Keep all medicines where children cannot reach them.
Side Effects of Antihistamines
Ask your provider if antihistamines are safe for you or your child, what side effects to watch for, and how antihistamines may affect other medicines you or your child take.
- Antihistamines are thought to be safe for adults.
- Most antihistamines are also safe for children older than 2 years old.
- If you are breastfeeding or pregnant, ask your provider if antihistamines are safe for you.
- Adults who take antihistamines should know how the medicine affects them before driving or using machinery.
- If your child is taking antihistamines, make sure the medicine is not affecting your child's ability to learn.
There may be special precautions for using antihistamines if you have:
- Diabetes
- Enlarged prostate or problems passing urine
- Epilepsy
- Heart disease or high blood pressure
- Increased pressure in the eye (glaucoma)
- Overactive thyroid
Side effects of antihistamines may include:
- Changes in vision, such as blurry vision
- Constipation
- Decreased appetite
- Dizziness
- Drowsiness
- Dry mouth
- Feeling nervous, excited, or irritable
- Increased difficulty urinating if you already have some difficulty urinating
When to Call the Doctor
Contact your provider if:
- Your nose is irritated, you are having nosebleeds, or you have any other new nasal symptoms
- Your allergy symptoms are not getting better
- You are having trouble taking your antihistamines
References
Corren J, Baroody FM, Togias A. Allergic and nonallergic rhinitis. In: Burks AW, Holgate ST, O'Hehir RE, et al, eds. Middleton's Allergy: Principles and Practice. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 40.
Dykewicz MS, Wallace DV, Amrol DJ, et al. Rhinitis 2020: A practice parameter update. J Allergy Clin Immunol. 2020;146(4):721-767. PMID: 32707227
Seidman MD, Gurgel RK, Lin SY, et al. Clinical practice guideline: allergic rhinitis. Otolaryngol Head Neck Surg. 2015;152(1 Suppl):S1-S43. PMID: 25644617
Version Info
Last reviewed on: 5/20/2024
Reviewed by: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
