What are Antihistamines?


There are many different antihistamines on the market, but not all are equal. Some are more effective and some can have dangerous side effect profiles.  None treat the root cause of allergies but can provide relief from allergic symptoms.

While the early ‘first generation’ antihistamines are known to cause drowsiness among other unwanted conditions, new second and third generation antihistamines are raising the bar with their potent and non-drowsy properties. Alternative allergy treatments that have longer term results are also rising in popularity.

What are antihistamines?

Antihistamines manage the symptoms of allergies. They can come as pills, chewable tablets, capsules, liquids, eye drops, sublinguals and injections. 

The body experiences an allergic reaction when a substance (allergen) is recognised as harmful, even though it isn’t. 

In response, the body overreacts by creating histamine, the chemical that fights disease and infection. This only causes discomfort and potential danger—the symptoms of allergies.

Antihistamines can treat common allergy symptoms by blocking histamine receptors. They help relieve:

  • Congestion
  • Itchy and watery eyes
  • Sneezing
  • Runny nose
  • Scratchy throat
  • Skin rashes
  • Hives

First generation antihistamines

First generation antihistamines, although effective and inexpensive, are no longer recommended because of their adverse side effects. 

These older antihistamines have active ingredients that are non-specific—they block other receptors, not just those that interact with histamine.  They also cross the blood-brain barrier, impairing the central nervous system, which commonly causes:

  • Sedation 
  • Lack of concentration
  • Poor sleep quality
  • Dry eyes
  • Dry mouth 
  • Dizziness

These significant side effects can be detrimental to a child’s school performance and to an adult’s ability to work and drive.

Common first generation antihistamines include:

  • Diphenhydramine (Benadryl)
  • Chlorpheniramine (Chlor-Trimeton)
  • Brompheniramine (Dimetane)
  • Doxylamine (Unisom)
  • Promethazine (Phenergan)
  • Triprolidine (Triafed)

Second and third generation antihistamines

Newer versions of antihistamines are non-sedating, safer, have a faster onset and last longer. Unlike first generation antihistamines that are taken every 4 to 6 hours, many of the newer alternatives only need to be taken once a day. 

Their active ingredients only block histamine receptors and they rarely cross the blood-brain barrier, allowing a smaller side-effect profile. They also interact with fewer drugs. 

However, there are still some side effects to be aware of, including:

  • Headache
  • Cough
  • Sore throat
  • Abdominal pain
  • Nausea or vomiting

Common second and third generation antihistamines include:

  • Loratadine (Claratyne)
  • Cetirizine (Zyrtec)
  • Fexofenadine (Allegra & Telfast)
  • Desloratadine (Clarinex)

A long term alternative: allergy shots

Allergy shots, a form of immunotherapy, are the closest treatments available for curing allergies. Instead of blocking histamine, these stop or reduce allergy attacks by injecting tiny amounts of an offending allergen into the patient. 

This allows the body to build tolerance to the allergen, causing allergy symptoms to diminish over time.   

This method of treatment does not produce side effects on the central nervous system like antihistamines. However, patients can experience localised allergic reactions that usually clear up quickly.

If you are currently experiencing a severe allergic reaction, call emergency now. If you’re experiencing the irritating symptoms of an allergy, contact your GP or find an allergist with Gamma Allergy. They can help you identify allergy triggers and point you toward a suitable antihistamine or alternate allergy treatment.