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.