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November 14, 2010

Zyrtec (OTC) and Xyzal (Rx) Contain the SAME Active Ingredient? Prilosec (OTC) and Nexium (Rx) Too!

If zyrtec and xyzal (allergy medications) both contain the same active ingredient (cetirizine), why are they "different" and one is sold over-the-counter (zyrtec) and the other available only by prescription (xyzal)? This is also true for reflux medication prilosec (OTC) and nexium (prescription) which contain the same active ingredient omeprazole.

It's because of mirror images of the same active ingredient molecule... with one being biologically active and the other inactive.

In nature, there is a "doppelganger" of sorts for every protein, compound, molecule, and even drug ingredients. This doppelganger is what is known as a stereoisomer and is an exact mirror image copy of the original. As such, they look the same, but are actually very different, just like a pair of hands. Though both hands appear to be an exact copy of the other, one is left and the other is right and that's why you can't wear a right glove with a left hand.

Still confused? Take a look at the picture below...

The two structures are an EXACT replica of each other with the colored balls in the exact same location relative to each other.

NOW... take a look at this picture below:

The two structures are an exact MIRROR image of each other and though they appear to be the same, the colored balls cannot be overlain on top of each other due to different relative locations.

What does this have to do with medications? Well in nature... ALL living things contain and utilize only the LEFT-handed structure (also known as L-isomer). The right-handed structure (or R-isomer) either is inactive biologically or in some cases, even poisonous. (Rarely, the R-isomer is the biologically effective structure.)

ZYRTEC contains the active ingredient cetirizine, but both the L and R-isomers.
XYZAL also contains the active ingredient cetirizine, but only the biologically active L-isomer.

Same goes for PRILOSEC which contains the active ingredient omeprazole, but both isomer shapes. NEXIUM contains only the L-isomer shape.

Now wait a minute... if you look at the box for NEXIUM, the active ingredient is esomeprazole and not omeprazole. Note that depending on the convention used, L-isomer may be called S-isomer; R-isomer may be called D-isomer. SO... "S"-isomer omeprazole is Nexium. "S"omeprazole sounds like esomeprazole.

Same goes for xyzal which has as it's active ingredient levocetirizine and not cetirizine. "Levo" is latin for left (from laevus). So L-ceterizine when not abbreviated turns into levo-cetirizine.


Now why in the world would a drug company sell a product that contains an inactive stereoisomer when it is a known fact that only the L-isomers are biologically active?

It's because of money...

A patent on a drug only lasts 10 years. After that, it goes off patent (generic) and other companies can start making and selling it. However, by making and selling a "dirty" version first (containing both L and R-isomers), they can reserve for a later time the "purified" version (L-isomer only) and get another 10 years of patent protection after the dirty version goes off-patent.

So when you see a commercial, read literature, or talk with a sales representative that xyzal is more effective than zyrtec, that nexium is more effective than prilosec... it IS true... but only because xyzal and nexium contain only the purified mirror-image of the active ingredient.

This story is played out over and over again... sometimes with horrific consequences. Most notoriously, there was the drug thalidomide sold in the 1950s...

Thalidomide was a widely prescribed sedative drug which was later found to cause horrible birth defects... well... only the S-isomer caused the birth defects. The R-isomer did not... But guess what? The thalidomide that was sold contained both isomers.
Fauquier blog
Fauquier ENT

Dr. Christopher Chang is a private practice otolaryngology, head & neck surgeon specializing in the treatment of problems related to the ear, nose, and throat. Located in Warrenton, VA about 45 minutes west of Washington DC, he also provides inhalant allergy testing/treatment, hearing tests, and dispenses hearing aids. Google+ Christopher Chang, MD Bio


  1. This is an extremely well-written article. The differences between isomers is well explained and I received all the information I needed and more from this article. Thank you so much, Dr. Chang--you have a real talent (not just skill) for teaching.

  2. Agreed! Thank you for the well-written explanation!

  3. A real eye-opener thank you Dr.Chang! I agree with the first poster you made the explanation really understandable a job well done. I was given Prilosec and had a bad reaction to it. I am allergic.Now I take Nexium without any problems. I never knew until now that they were the same medicines only in different forms.

  4. Zyrtec does nothing for me but Xyzal helps tremendously.

    1. Same for me. But giving Zyrtec a try per my OB's suggestion now that I am pregnant. So far, not doing anything ....

    2. Dr. Chang, it wasn't that Thalidomide was sold as a racemic mixture, the point was moot. While you are correct that the (R) enantiomer of thalidomide is effective against morning sickness and the (S) enantiomer is teratogenic, the drug racemizes in vivo, and therefore was not physiologically available as only the (L) stereoisomer during its period of use. It seems plausible that the (L) stereoisomer could be chemically altered and stabilized to prevent racemization and retain its usual pharmacologic activity, but we now have Zofran!

  5. I am curious why the author leaves out the facts surrounding isolation of the L isomers. Cost associated with the lab work, testing costs (all phases) failure rates post launch etc. Once you declare research the patent clock starts. Isolating the metabolite aka Atarax to Zyrtec and the isomers Zyrtec to Xyzal takes time and research money. You're still not guaranteed to succeed and if it's a bust, you're out hundreds of millions if not billions. It's a little more involved than greed.

    1. The government subsidizes pharmaceutical companies research, etc. So, they have very little of the costs involved in new drugs, but they keep all the massive profits. Drugs are very often pennies to produce, but cost us, the consumers, hundreds of dollars. Or cost the insurance companies that much. Pity the poor people without insurance who have to pay the highway robbery, and for me personally, my pet's meds are VERY expensive. I KNOW they don't have to be. For instance, Flovent (life-saving oral medicine for asthma) in the USA is $300 each (for a month's supply!), but I can get it out the country for $30/each. It's BigPharma's GREED that makes it so expensive. I find it disgusting & reprehensible. That's why our "health care" system is broken - BigPharma's GREED.

    2. Doesn't cost pennies to develop and get fda approval but certainly not the millions they pull in. The racemates are all about cold hard cash, indeed.

  6. Thanks for s clear explanation. I assumed something with both L and R not just would be better! Now I understand.


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