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?
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.
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…
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:
On the other hand, negative side effects of fexofenadine include:
Tracy decides that she's going to try Zyrtec. She has no plans to get pregnant. She and her allergist will watch for potential heart rhythm issues and will evaluate whether she feels too tired on Zyrtec. Tracy wants to avoid the muscle aches and headaches that Allegra may cause, and she loves her daily glass or orange juice! After a week on Zyrtec, Tracy is back in her garden, digging and planting while free of itchy hives!
Antihistamines are medications that work by blocking a chemical messenger called histamine, which is a trigger of allergy symptoms in the body. Cetirizine and fexofenadine are second-generation antihistamines, which are less sedating. Both block histamine release from mast cells to reduce reactions to allergens, relatively harmless trigger substances like dust and pollen. Both may cause blurred vision, dizziness, dry mouth, and nausea or vomiting.
Here are some of their differences:
Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.
To unlock this lesson you must be a Study.com Member.
Create your account
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.
Already a member? Log In
Back