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Which of the following is commonly known as “Z-Drug”?

a. Omeprazole
b. Olanzapine
c. Zolpidem
d. Nizatidine
e. Doxylamine

Answer: (c) The Z-drugs, which include Eszopiclone (Lunesta), Zolpidem (Ambien), and Zaleplon (Sonata), are benzodiazepine receptor agonists.  That means they work in a similar way to the benzodiazepine drugs inside the brain.  They are GABA agonists meaning they somewhat mimic the action of gamma-Aminobutyric acid, the inhibitory neurotransmitter and thereby induce sleepiness.
These drugs are sometimes referred to as non-benzodiazepine hypnotics or just non-benzodiazepines.  That's a dumb name, if you ask us.  Too unspecific and vague, especially if you are not in the context of sleep medicine. 

Further, even within sleep medicine, there are compounds that are non-benzodiazepine hypnotics that would not be considered part of this class: antihistamines and Ramelteon, for instance.

One problem is that the chemists don't have a category that these drugs all fall into which is narrow enough to signify what medical practitioners are talking about.  These drugs are in the categories pyrazolopyrimidines, imidazopyridines or cyclopyrrones, but they are not all in the same category.
Therefore, we prefer the term Z-drugs.  The generic names for these drugs all contain the letter Z, and it is as good a name as any.