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When a person has an allergic reaction, they may experience symptoms such as nasal congestion, sneezing, and itchy skin. Antihistamines can help ease these symptoms.

This article will look at how antihistamines work, as well as the differences between first-, second-, and third-generation antihistamines and their side effects.

a man sneezing and could do with taking a antihistaminesShare on Pinterest
A person may take antihistamines to treat allergic rhinitis.

A person experiences an allergic reaction when they come into contact with a harmless substance that their body interprets as an invading pathogen.

Allergens are substances that trigger allergic reactions, and they include anything from pet dander and pollen to specific proteins found in foods.

When an allergen enters a person’s body or touches their skin, cells in the immune system release histamines, which bind to specific receptors located on cells found throughout the body.

Once histamines bind to these receptors, they trigger several typical allergic reactions, such as expanding the blood vessels and causing the smooth muscle tissues to contract.

Antihistamines refer to a type of medication that treats allergy symptoms, motion sickness, and some cold and flu symptoms. Antihistamines block H1 histamine receptors.

People can use antihistamines to treat allergic rhinitis, which causes inflammation in the nose.

Antihistamines can help alleviate a wide range of symptoms, such as:

  • congestion
  • sneezing
  • runny nose
  • itchy or watery eyes
  • itching
  • skin rashes
  • hives
  • nausea

There is a range of antihistamines available.

First-generation

First-generation oral antihistamines, including diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton), easily cross the blood-brain barrier and affect H1 receptors in the central nervous system (CNS).

H1receptors in the CNS help regulate the body’s sleep-wake cycle.

First-generation antihistamines have sedative properties. By binding to receptors in the CNS, first-generation antihistamines can impair cognitive and motor functions and cause drowsiness.

Other severe side effects associated with first-generation antihistamines include:

Other examples of first-generation antihistamines include:

  • brompheniramine (Dimetane)
  • carbinoxamine (Clistin)
  • clemastine (Tavist)
  • doxylamine (Unisom)
  • hydroxyzine (Atarax, Vistaril)
  • promethazine (Phenergan)
  • triprolidine (Triafed)

Although several types of first-generation antihistamines are available, some healthcare professionals may not recommend them if a newer version is available.

Second- and third-generation

Second- and third-generation antihistamines do not have sedative properties. According to an article in the journal National Institute of Diabetes and Digestive and Kidney Diseases, second- and third-generation antihistamines are less likely to cross the blood-brain barrier. This means they do not have such significant effects on the CNS as first-generation types.

These antihistamines are safe for adults and children over 12 years old to use.

According to the authors of one 2019 review, second- and third-generation antihistamines are safer and more potent than the first-generation types.

Examples of second- and third-generation antihistamines include:

  • bilastine (Bilaxten)
  • desloratadine (Clarinex)
  • loratadine (Claritin)
  • fexofenadine (Allegra)
  • rupatadine (Rupafin)

Over-the-counter antihistamines

People can purchase a variety of antihistamines at their local pharmacies. They are available in several different forms, such as tablets, gel capsules, nasal sprays, and eye drops.

Types of over-the-counter (OTC) antihistamines include:

  • Benadryl
  • Chlor-Trimeton
  • Claritin
  • Allegra
  • Tavist
  • Zyrtec

People can speak with a doctor if they need help deciding which type of antihistamine is right for them.

Prescription antihistamines

Some types of antihistamines are only available by prescription from a licensed healthcare professional.

Antihistamines that require prescriptions may contain higher concentrations of active ingredients than OTC types.

Other antihistamines may be prescription only because of the risk of adverse side effects.

Examples of antihistamines that require a prescription include:

  • azelastine (Astelin, Astepro, Optivar)
  • carbinoxamine (Palgic)
  • desloratadine (Clarinex)
  • hydroxyzine (Atarax, Vistaril)
  • levocetirizine (Xyzal)
  • promethazine (Phenergan)

Anyone taking antihistamines or any medication should check the label to see which active ingredients the medication contains to avoid the risk of an overdose. If a person overdoses on sedating antihistamines, they may experience cardiac abnormalities or seizures.

If a person suspects that they are experiencing an overdose, they should seek immediate medical attention or call the American Association of Poison Control Centers (AAPCC) for advice at 1-800-222-1222. The AAPCC helpline is open 24 hours and is toll-free.

Most people can safely take low doses of OTC or prescription antihistamines for short periods.

However, antihistamines can cause mild side effects in some people. These may include:

Severe side effects can include:

  • hives
  • a skin rash
  • difficulty breathing or swallowing
  • swelling in the face, mouth, or throat
  • delirium

Although rare, long-term use of non-sedating antihistamines (second- and third-generation) use may result in mild liver damage. However, terfenadine, which is one of the antihistamines associated with liver damage, is no longer available for clinical use.

It is safe for pregnant women to take most first- and second-generation antihistamines at low doses. However, taking high doses of promethazine, which is a first-generation sedating antihistamine, may cause adverse side effects during pregnancy.

Pregnant women should speak with their doctor before taking antihistamines. Doctors can provide appropriate dosage recommendations based on a person’s symptoms and health status.

Most healthcare providers recommend using second- or third-generation antihistamines to treat mild to moderate allergy symptoms, including congestion, watery eyes, and itchy skin.

People can still buy first-generation antihistamines. However, these forms can cause drowsiness and sedation.

People can choose between a wide range of antihistamines in drugs stores and online.

Parents and caregivers may want to consult a health care professional before giving an antihistamine to a child, especially if the child is 12 years old or younger.