Claritin loratadine vs Zyrtec cetirizine

Claritin (loratadine) and Zyrtec (cetirizine) are part of a family of second-generation antihistamines used for treatment of hay fever allergies and hives. This group of drugs also includes fexofenadine (Allegra) and azelastine (Astelin)

The advantage of Claritin and Zyrtec is they cause less drowsiness than older anti-allergy drugs like Benadryl or hydroxyzine. Cetirizine and loratadine are very similar. Both interact with few other drugs, but the drugs that cause adverse reactions when combined with Claritin are different than the ones that cause bad reactions with Zyrtec.

Another difference is Zyrtec, according to a 2014 study, is also more likely than Claritin to cause drowsiness as a side effect.

Hay fever (allergic rhinitis), hives (urticaria), and pollen-induced asthma are all reactions to histamines released by the body in response to an allergen (dust, pollen, or animal dander). Second-generation antihistamine drugs like Claritin and Zyrtec keep cells from interacting with histamine, thereby preventing allergy symptoms.

Histamine is a relatively simple, nitrogen-based compound manufactured in special white blood cells called mast cells. Histamine is a crucial messenger chemical used for all kinds of important physiological and neurological functions. The function a histamine molecule performs in any given physiological process depends not on the chemical itself, but how cell proteins latch onto and interpret it.

Researchers have identified four different histamine receptor proteins thus far, named H1 through H4. They each kick off drastically different processes when exposed to histamine. A lot of H1 receptor proteins are located on the outer membranes of nerve cells and blood vessel cells in the mucous membranes of the airways and gut - basically any tissue that has exposure to the outside environment. When, for example, pollen stimulates mast cells to release histamine, the histamine molecules latch onto the H1 receptor proteins, which causes capillaries to open, tissue to swell, and membranes to become more permeable to fluid. In people with allergies, the mast cells overreact to allergens and release way too much histamine, causing runny nose, itchy eyes, sneezing, and inflamed airways.

Loratadine and cetirizine are H1 receptor antagonists. This means each molecule of the medication has a shape and chemical properties that make it fit into the H1 receptor, locking out the histamine without kicking off the inflammation reaction.

Older antihistamines like hydroxyzine are blunt instruments compared to second-generation ones like Claritin and Zyrtec. As mentioned before, histamines are crucial for all sorts of neurological functions, including maintaining normal levels of alertness and wakefulness. Older antihistamines helped with allergy symptoms, but they also blocked H1 receptors in the brain, causing drowsiness.

Molecules of loratadine and cetirizine are shaped in such a way they can't fit through the membranes that separate the bloodstream from brain tissue, meaning Claritin and Zyrtec cause less drowsiness that older antihistamines. According to a 2014 study, however, second-generation antihistamines -- Zyrtec especially -- can affect mood and cause daytime drowsiness more often than doctors initially believed. So, just because the newer medications are more precise doesn't mean they're without side effects.

Claritin tends to have fewer side effects than Zyrtec. They include headache, drowsiness, fatigue, dry mouth. Conversely, loratadine can cause nervousness and difficulty sleeping in some patients.

Zyrtec may cause all those same side effects, plus nausea and sore throat. Daytime sleepiness is much more prevalent in people taking cetirizine; it happens in about 14% of patients.

Rare but serious side effects of cetirizine include allergic reactions, seizures, fainting, and low blood pressure.

Erythromycin, cimetidine (Tagamet), and ketoconazole (Nizoral) increase the blood concentration of loratadine by making it harder for the body to break down. This may result in increased side effects.

St. John's wort, carbamazepine (Tegretol, Tegretol XR , Equetro, Carbatrol, Epitol, Teril), and rifampin reduce blood levels of loratadine, making it less effective.

Theophylline (Theo-Dur, Respbid, Slo-Bid, Theo-24, Theolair, Uniphyl, Slo-Phyllin) reduces the body's ability to breakdown cetirizine by 16%, increasing blood levels of the medication. Drugs that cause drowsiness may add to drowsiness resulting from cetirizine.

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Reviewed on 3/22/2017 References REFERENCE:

"Structure-Function of the G-protein-Coupled Receptor Superfamily"
V. Katritch, V. Cherezov, and R.C. Stevens
Annual Review of Pharmicological Toxicology; Jan, 2013

"Molecule of the Month: Histamine"
University of Bristol School of Chemistry, 2008

FDA Prescribing Information

"Histamine Compound Summary"
PubChem

"Loratidine Compound Summary"
Pubchem

"Cetirizine Compound Summary"
Pubchem

"HRH1 histamine receptor H1 [Homo sapiens (human)]"
Gene Summary
National Center for Biotechnology Information

"Assessment of the effects of antihistamine drugs on mood, sleep quality, sleepiness, and dream anxiety"
Pinar Guzel Ozdemir, Ayse Serap Karadag, Yavuz Selvi, Murat Boysan, Serap Gunes Bilgili, Adem Aydin, and Sevda Onder
International Journal Of Psychiatry In Clinical Practice, 2014