Healthy Living

What Are Histamine Blockers and How Do They Work?

What Are Histamine Blockers and How Do They Work?

What are histamine blockers?

Histamine blockers also called as antihistamines are medications that block or reduce histamine to stop the symptoms of allergy. 

Histamine is a chemical that is released by a type of white blood cell called mast cell in response to allergies or injuries. The cells that are found in the blood vessels, brain, skin, stomach, and lungs have proteins called histamine receptors that bind to histamine to produce an inflammatory response. 

The symptoms of this inflammatory response may include:

Histamine blockers prevent histamine from binding to its receptors to reduce or inhibit allergy symptoms. These medications affect three types of histamine receptors, which are H1-, H2-, and H3-receptors. The histamine receptors that are widespread in the body are H1-receptors, while H2- and H3-receptors are largely found in the stomach and brain, respectively. 

  • H1-blockers: Help relieve allergic rhinitis, skin rash, sinus congestion, and severe allergic reactions. 
  • H2-blockers: Decrease the amount of acid that is produced by the cells in the stomach lining. It helps relieve conditions such as gastroesophageal reflux disease or GERD
  • H3-blockers: These medications are still being studied, especially in conditions that involve the human brain and sleep. 

Allergies can be relieved using different types of medications, which include allergy shots and steroids. However, in most cases, the initial medications used to help relieve allergy symptoms are antihistamines.

There are also antihistamines that are sometimes used to help relieve nausea, vomiting, motion sickness, anxiety, dizziness, and Parkinson's disease. 

What are the available types of antihistamines?

Antihistamines are available in different forms. They can be in the form of eye drops, nasal sprays, capsules, tablets, and liquids. These medications are also available over-the-counter, although some of them are only available by prescription. 

Some antihistamines are also mixed with other types of medications, such as decongestants and pain relievers. However, it would not be a good idea to use combination drugs if you only have a runny or stuffy nose. When buying OTC antihistamines, remember to buy medication for the symptoms that you only have.

There are two types of antihistamines that you can buy without a doctor's prescription. They are the first-generation and second-generation antihistamines. Both of these types are effective when it comes to relieving symptoms of allergy. Sometimes, first-generation antihistamines are used in over-the-counter medications for colds. 

The following are some examples of over-the-counter antihistamines:

  • First-Generation Antihistamines: Diphenhydramine (Benadryl, Sominex, Nytol), clemastine (Tavist), brompheniramine (Dimetane, Children’s Dimetapp Cold), chlorpheniramine (Chlor-Trimeton, Actifed Cold and Allergy), dimenhydrinate (Dramamine), and doxylamine (Tylenol Cold & Cough Nighttime, Vicks NyQuil)
  • Second-Generation Antihistamines: Cetirizine (Zyrtec), loratadine (Claritin, Alavert), and fexofenadine (Allegra)

Antihistamines that are only available by prescription include:

  • Desloratadine (Clarinex)
  • Hydroxyzine (Atarax, Vistaril)
  • Carbinoxamine (Palgic)
  • Cyproheptadine (Periactin)
  • Levocabastine oral (Xyzal)
  • Azelastine nasal sprays (Astepro, Astelin)
  • Azelastine ophthalmic (Optivar)
  • Emedastine ophthalmic (Emadine)
  • Levocabastine ophthalmic (Livostin)

Ophthalmic antihistamine drops help relieve the symptoms of eye allergies, such as watery, itchy, and red eyes. 

Side Effects of Antihistamines

Drowsiness is a common side effect of older antihistamines. For this reason, newer antihistamines may be a better drug of choice for some people. Below are some of the common side effects of antihistamines:


Antihistamines that can cause drowsiness may be taken before going to bed and not during the day, particularly when driving or using machinery at work. Drug interactions may also happen when antihistamines are taken along with other types of medications. Inform your healthcare provider regarding all prescription, over-the-counter medications, herbal medications, or dietary supplements that you may be taking before using antihistamines. Always check medicine labels before taking medications for your allergy. 

Moreover, consult your healthcare provider before taking antihistamines, especially if you are pregnant or nursing or have any of the following conditions:

During the first trimester of pregnancy, hydroxyzine is not recommended since this drug has shown congenital defects in animal studies, particularly when it is given in higher doses than the typical dose given to humans.  

Fexofenadine and desloratadine are types of antihistamines that have not been extensively studied in women who are pregnant. The following antihistamines have not shown sufficient evidence of causing problems or congenital defects in animal studies and have not been studied in pregnant women as well:

  • Diphenhydramine
  • Loratadine
  • Brompheniramine
  • Clemastine
  • Cetirizine
  • Cyproheptadine
  • Chlorpheniramine
  • Azatadine
  • Dimenhydrinate
  • Doxylamine
  • Dexchlorpheniramine

When it comes to breastfeeding, small amounts of antihistamines may pass into breast milk. Antihistamines are often not recommended to use while breastfeeding since infants tend to be more prone to experiencing drug side effects, such as irritability and unusual excitement. 

Some side effects of antihistamines also tend to get worse when alcohol is consumed. For this reason, alcohol consumption should be avoided when you are taking antihistamines. 

Consuming grapefruit or grapefruit juice may also affect how antihistamines work in the body. Speak with your healthcare provider if this issue raises a concern. 

Choosing OTC Antihistamines

When choosing OTC antihistamines, there are also some things to consider. Allergies can be relieved by a number of over-the-counter medications, including branded antihistamines, such as:

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

Check with your doctor or ask a pharmacist if you are still unsure which type of antihistamine to take for your condition. When taking antihistamines, also make sure that you are not taking other medications that belong to the same drug class to avoid taking excessive amounts of a particular drug. 

Some antihistamines may only work for 4-6 hours, while others may last up to 24 hours. Read the medicine label carefully and make sure that you know the correct dosage of the antihistamine you are taking. There are antihistamines that can cause less drowsiness than other types of histamine blockers, and they include:

  • Loratadine (Claritin)
  • Desloratadine (Clarinex)
  • Cetirizine (Zyrtec)
  • Fexofenadine (Allegra)

Antihistamines should also be stored at room temperature and away from heat, moisture, high temperature, and direct light. Moreover, antihistamines and all other medications should be kept away from children's reach.