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Reviewed by: HU Medical Review Board | Last Reviewed: September 2022

Antihistamines are drugs used to treat the symptoms of allergic reactions. Antihistamines have been around for more than 80 years and come in many different forms. Antihistamines work by blocking the effects of histamines. These are chemicals made by the body during an allergic reaction.1-6

Antihistamines taken by mouth are often used to treat hay fever, itchy eyes or skin, or mild food reactions. These drugs are often the first choice for treatment if avoiding the allergen has not worked.1-6

How do antihistamines work?

Things that cause an allergic reaction are called allergens. Pollen, dust mites, pet dander, food, and latex are some of the most common allergens. An allergen is an otherwise harmless substance that triggers the immune system to react in some people.2,4

When the body has an allergic reaction, it causes allergy cells to release histamine. Histamines cause allergy symptoms such as:2,4

  • Swelling
  • Inflammation
  • Itching
  • Rash
  • Runny nose
  • Sneezing
  • Watery eyes

Antihistamines are drugs that block histamines in the body. In turn, this reduces the symptoms of an allergic reaction.2,4

Types of antihistamines

These drugs may be taken several ways, including as:1-3

  • Dissolvable or chewable tablets
  • Capsules
  • Nasal sprays
  • Eyedrops
  • Liquids, creams, gels, and sprays

Some antihistamines treat an allergic reaction on a specific part of the body, like eye drops or a skin spray. Pills treat the entire body. Some are available over-the-counter (OTC), and some only by prescription. 2,5,6

Older antihistamines

First-generation antihistamines were developed in the 1930s. They act on histamine receptors in the body. Some of these drugs may cause drowsiness and fatigue. This happens because the drug crosses the blood-brain barrier.5

Examples of older over-the-counter (OTC) antihistamines include:1,3,4

  • Brompheniramine (Dimetane®)
  • Chlorpheniramine (Chlor-Trimeton®)
  • Clemastine (Tavist™)
  • Diphenhydramine (Benadryl®)
  • Hydroxyzine (Atarax®, Vistaril®)
  • Promethazine (Phenergan®)

Newer antihistamines

Second-generation antihistamines were developed in the 1980s. Third-generation antihistamines are even newer. Some doctors consider these drugs safer than the older ones. These also act on histamine receptors in the body but do not generally penetrate the brain. This means these drugs are less likely to cause drowsiness and have fewer side effects.1,3,4

Newer oral antihistamines include:1,4

  • Cetirizine (Zyrtec®)
  • Desloratadine (Clarinex®)
  • Loratadine (Alavert®, Claritin®)
  • Fexofenadine (Allegra®)
  • Levocetirizine (Xyzal®)

These drugs may still cause drowsiness if taken at a higher than the recommended dose.

Antihistamine nasal sprays

Antihistamine nasal sprays help relieve sneezing, itchy or runny nose, and postnasal drip. Side effects of antihistamine nasal sprays include a bitter taste, drowsiness, or fatigue. Antihistamine nasal sprays require a prescription. Examples include:1,5

  • Azelastine fluticasone (Dymista®)
  • Azelastine hydrochloride (Astelin®, Astepro®)
  • Olopatadine (Patanase®, Patanol®)

Antihistamine eye drops

Antihistamine eye drops relieve itchy, red, and swollen eyes. These drops are available over-the-counter and by prescription. Common side effects of eye drops include headache and dry eyes.5

Some eye drops include:5

  • Azelastine (Optivar®)
  • Emedastine (Emadine®)
  • Ketotifen (Alaway®)
  • Olopatadine (Patanol®, Pataday®, Pazeo™)
  • Pheniramine (Opcon-A®, Visine-A®)

What are the possible side effects of antihistamines?

Side effects can vary depending on the specific drug you are taking However, there are some side effects common to all antihistamines, including:1,3,4,6

  • Drowsiness
  • Slowed thinking
  • Poor sleep
  • Dry mouth
  • Dizziness
  • Low blood pressure
  • Rapid heartbeat

Doctors recommend people avoid driving or operating machinery when taking first-generation antihistamines.1,3

Second- and third-generation antihistamines are less likely to cause drowsiness at the basic dose or interact badly with other medicines.1,3,6

These are not all the possible side effects of antihistamines. Talk to your doctor about what to expect or if you have any changes that concern you during treatment.

Things to know about antihistamines

When taking antihistamines, be careful when performing activities that require alertness. This includes driving or operating heavy machinery. Your doctor may recommend taking an antihistamine before bed if it makes you drowsy.2,3,5

Antihistamines are generally safe for adults, including people who are pregnant. Some types are safe for children. Your doctor can recommend one that is right for you.3-5

Before beginning treatment with an antihistamine, tell your doctor about all your health conditions and any other drugs, vitamins, or supplements you take. This includes over-the-counter drugs.

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