Difference Between Antihistamine and Decongestant

If you have a cold or a runny nose, you are likely to take an Antihistamine or a decongestant. These  act as allergy relief medicines or to help relieve nasal congestion, colds causing a runny nose, sneezing, hay fever, etc. To some people they may sound fairly similar in nature but they are actually quite different in the way they work to help relieve the symptoms of various conditions and their mechanisms.

Antihistamine vs Decongestant  

The main difference between antihistamines and decongestants is that antihistamines work by blocking the actions of histamines in your body, while decongestants work by reducing the swelling in the blood vessels present in your nose to help relieve blocked/stuffed noses. 

Antihistamine vs Decongestant

Antihistamines are the class of drugs that help prevent or treat the symptoms of allergies including hay fever, allergic reactions to insect bites, pollens, etc. They do so by blocking the actions of naturally occurring histamines. 

On the other hand, decongestants help you by relieving the symptoms of stuffy noses or blocked nose you get by allergic reactions, colds, flu, sinusitis, etc. They open up the airways in your nose by reducing the swelling of the blood vessels present in it which improves the air flow. 

Comparison Table Between Antihistamine and Decongestant 

Parameters of Comparison Antihistamines Decongestants 
Mechanism They reduce the effect of histamines by blocking their mechanisms. They constrict the muscles present in the blood vessels in your nose. 
Receptors Antihistamine blocks H1 or H2 cell receptors. Decongestants effect the alpha-adrenergic receptors. 
Uses These are used to treat allergic reactions, colds, stomach ulcers, GERD, motion sickness, etc. They are used to reduce the effects of allergic reactions, anaphylactic reactions, catarrh, sinusitis, etc. 
Available forms Available in tablets and liquids. Mostly available in nasal sprays, nose drops, and tablets. 
Side effects Blurred vision, dizziness, confusion, dry mouth, trouble urinating, drowsiness, diarrhea, etc. Increased blood pressure, restlessness, dizziness, anxiety, insomnia, heart palpitations, headaches, increased heart rates, etc. 

What is Antihistamine? 

Your body releases histamines when you come near anything that may cause an allergic reaction like pollens, insects, animal hair etc. Histamines cause an allergic reaction that expands your blood vessels and causes swelling to your skin, which helps in your body’s protection. 

Histamines cause various symptoms like stuffed noses, itchy and watering eyes, skin rashes, sneezing, etc. Antihistamines work towards blocking the action of these histamines or to at least reduce their after-effects. 

Antihistamines blocks histamines from blinding at the H1 or H2 cell receptors that are present on the cell membrane. At the H1 receptor, antihistamines compete with the naturally occurring histamines by occupying the receptors beforehand. The antihistamines that work on the H2 receptors, work by blocking the receptors so that the histamines can’t bind with them. 

They are used to treat various conditions and their symptoms like hay fever, allergic reaction, GERD, stomach ulcers, etc. Their side effects may include drowsiness, blurred vision, confusion, diarrhea, headaches, etc. 

Some examples of antihistamines are promethazine, cimetidine, loratadine, diphenhydramine, Benadryl, etc. 

What is Decongestant? 

Decongestants are medicines that work by reducing the swelling in your nose’s blood vessels by causing constriction in the muscles present. They decrease the blood flow to the mucous membranes present in the nose by impacting the alpha-adrenergic receptions and binding with them.  

This causes muscle contraction which causes a decreased blood flow resulting in the reduction of the inflammation of the tissues found. This helps you breath better by opening up the air flows in your nasal cavities. 

These medicines help in the alleviation of inflammation related symptoms as they are vasoconstrictors. They reduce the secretions in the nasal passageways that occur because of allergic reactions or colds which cause inflammation and excretion of excess mucus.   

The side effects of decongestants include increased blood pressure, heart palpitations, anxiety, insomnia, headaches, increased heart rate, etc. 

Some examples of decongestants are epinephrine, phenylephrine, ephedrine, pseudoephedrine, Sudafed, Adrian nasal spray, etc. Epinephrine is used for people experiencing anaphylactic reactions or shocks. 

Main Differences Between Antihistamine and Decongestant 

  1. Antihistamines block the actions of histamines to relieve the symptoms of allergic reactions and decongestants reduce the swelling in the blood vessels to help make breathing easier. 
  2. Antihistamines block the H1 or H2 cell receptors during their mechanism whereas decongestants affect the alpha-adrenergic receptors which contracts the muscles and reduces the inflammation present near your nasal passageways and helps improve your air flow. 
  3. Antihistamines are most commonly found in tablet or liquid form and decongestants are most often found in nasal sprays or drops and tablets. 
  4. Side effects of antihistamines include blurred vision, dry mouth, confusion, drowsiness, headaches, diarrhea, dizziness, trouble while urinating, etc. and side effects of decongestants include insomnia, dizziness, anxiety, increased blood pressure, increased heart rates, heart palpitations, etc. 
  5. Antihistamines help in the treatment of allergic reactions, GERD, stomach ulcers, prevent motion sickness, stomach ulcers, etc. Decongestants help reduce the effects of anaphylactic reactions, allergic reactions, sinusitis, etc. 


Antihistamines and decongestants help your with allergic reactions and colds and various other conditions through different means. Antihistamines blocks the H1 and H2 receptors to stop the actions of histamines which helps relieve the symptoms of allergic reactions. 

Decongestants shrink the blood vessels by influencing the alpha-adrenergic receptors which reduces the swelling present in your nasal passageways. This improves the airflow and relieves the pressure in your nostrils. 

If you are only suffering from cold symptoms or sneezing or just a stuffy or blocked nose, decongestants may be more helpful for you whereas if you are experiencing these symptoms because of a simple allergy, then antihistamines may be more helpful as they will help stop the functioning of the histamines released by your body. 

Both of these medicines have various side effects that need to be monitored and the medications will have to be altered accordingly.  


  1. https://www.nhs.uk/conditions/antihistamines/
  2. https://www.nhs.uk/conditions/decongestants/
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