Cetirizine vs. Fexofenadine

Lesson Transcript
Kaitlin Baker

Kaitlin has taught nursing students and has a master's degree in nursing leaderhsip, as well as a bachelor's degree in English literature.

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Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

In this lesson, learn about cetirizine (Zyrtec) and fexofenadine (Allegra) and their roles in treating allergy symptoms. Compare and contrast each drug in terms of how they work, what conditions they treat, and associated side effects. Updated: 03/11/2021

Allergy Season

Tracy has wanted to work in her garden all spring, but when she stays in her backyard for more than a couple of minutes, allergy symptoms come on with a vengeance. Her eyes itch, post-nasal drip intensifies, and red, itchy hives appear on her arms and legs. Tracy goes to her allergist and tells him of the symptoms, hoping that something can be done to help her tolerate being outdoors so she can plant her garden.

Tracy's allergist, Dr. Chang, wants to prescribe an antihistamine. These medications work by blocking a chemical messenger called histamine, which is a trigger of allergy symptoms in the body.

Dr. Chang tells Tracy that there are two antihistamine options he thinks they should choose between: cetirizine (brand name Zyrtec) and fexofenadine (brand name Allegra). But what are the similarities and differences between these medications in terms of how they work, what they are used for, and their side effects?

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  • 0:04 Allergy Season
  • 1:00 Histamine & Antihistamines
  • 2:01 How Zyrtec & Allegra Work
  • 3:34 Side Effects
  • 5:19 Lesson Summary
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Histamine & Antihistamines

Antihistamines work by blocking the action of histamine. But what does histamine do? When working correctly, histamine is part of the body's natural defense mechanism (the immune system) against harmful foreign invaders. People who have allergies have immune systems that over-react to substances that may be harmless to other people, such as dust, pollen, grasses, and mold. These substances are called allergens.

Histamine is stored in mast cells in nearly all of the body's tissues. When mast cells sense danger, they release histamine, triggering a chain reaction involving increased blood flow and the release of other inflammatory chemicals. This is what cause redness, swelling, itching, and other allergic symptoms. In severe cases, they can cause anaphylaxis (the inability to breathe). Antihistamines work by blocking the receptors that histamine tries to attach to during an allergic reaction, effectively stopping or reducing allergy symptoms.

How Zyrtec & Allegra Work

Both Zyrtec and Allegra are considered second-generation antihistamines. The earliest antihistamine medications are called first-generation antihistamines and include diphenhydramine (Benadryl), which is more sedating than Zyrtec and Allegra.

Second-generation antihistamines do not readily cross the blood-brain barrier, making them less effective for symptoms like nausea but also less likely to cause central nervous system side effects such as sedation.

Zyrtec and Allegra both have longer half-lives (how long it takes for half the drug to be completely out of one's system), meaning that their effects last longer and they do not have to be taken as frequently as the first-generation drugs.

Allegra does have a longer half-life than Zyrtec, almost twice as long, so it usually is always taken just once a day, while Zyrtec can be taken up to twice daily if needed. Both drugs start working relatively quickly once taken. Allegra starts working within two hours; Zyrtec within one.

In general, Zyrtec and Allegra work in a very similar manner. So far, so good for Tracy. Which medication should she choose?

Tracy asks if one medication or the other is better for her bothersome hives. The allergist tells her that there are some differences in the specific symptoms each drugs is most helpful for. Zyrtec is a little more specific when it comes to treating hives and itchy, runny nose.

One point for Zyrtec in Tracy's book! Hives are her most bothersome symptom. Now to consider side effects…

Side Effects

Both Zyrtec and Allegra may cause the following: blurred vision, dizziness, dry mouth, and nausea or vomiting. These are usually minor and often go away in time.

The negative side effects of cetirizine include:

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Additional Activities

Word Scramble Activity

In this activity, you'll check your knowledge regarding the similarities and differences between cetirizine and fexofenadine.


For this activity, study the scrambled letters and unscramble or rearrange the letters to form a word or phrase that fits the given clues. To do this, you must right-click and print this page. With a pencil and an eraser, neatly write your answers in the appropriate blank spaces in the clues.

Scrambled Words

  6. IEVHS


  1. __________ are drugs used to alleviate the symptoms of allergies in the body.
  2. __________ is a first-generation antihistamine used to relieve allergy symptoms, such as sneezing and itching.
  3. Zyrtec is an over-the-counter drug that belongs under the generic name __________.
  4. __________, such as dust, pollen, and mold, are harmless substances capable of triggering an allergic reaction.
  5. Some studies showed that taking cetirizine can cause an increased risk of abnormal __________
  6. __________ are raised areas of the skin that itch intensely and are red in color.
  7. Drinking orange juice upon taking a __________ medication may cause a decrease in drug absorption and effectivity.
  8. Histamine is a __________ released by the body during allergic reactions.

Answer Key

  1. Antihistamines
  2. Benadryl
  3. cetirizine
  4. Allergens
  5. heart rhythms
  6. Hives
  7. fexofenadine
  8. chemical messenger

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