I think it has more to do with region than situation. For example, here in the U.S. (at least my part of it), we would never use the word "chemist" to mean pharmacist, though it is common elsewhere. For us, "chemist" is more often used to refer to scientists who research and create things. Pharmacists are limited to dispensing medications. For example, a chemist I am acquainted with develops plastics and glues for a prominent U.S. company. Another chemist I know develops and tests medical compounds, but does not dispense them. Out here we are also more likely to say "drugstore" then "dispensary." (In fact, I think I only know the word "dispensary" in that context because of travel and foreign movies.) We use "drugstore" to refer to any convenience store that also includes a pharmacy. Usually those stores sell a variety of other products -- snacks, toiletries, makeup, magazines, greeting cards, etc. It is less common to see a store dedicated only to selling medicine, but they do exist. They tend to be independently owned rather than chains, and we often call those stores a "pharmacy" rather than a "drugstore" (though the words are technically interchangeable). In a large drugstore, only the section of the store that dispenses prescribed medications is called a "pharmacy." I hope that's helpful. Someone else will have to chime in about how the words are used in other English-speaking countries (and possibly even other states).