Are the prepositions correct?
At the chemist's (or in a chemist??), at the drugstore, at the (or a??) pharmacy

Question

Answer

Usually 'the' would refer to a particular chemist, etc., but sometimes is used in instructions to mean whichever chemist is applicable  e.g.  'take this prescription to the chemist and get the medicine'  -  it could be any chemist.  It is equally correct to say  ''take this prescription to a chemist and get the medicine".

Answer

AT - is used for times.

At midnight," at noon",

Or contexts ex: " at work, "at school" especific places .

Answer

You may use both 'at' and 'in', depending on the context:
'She bought it at the chemist's'.
'I work in a chemist'.
'I'm in the drugstore' (emphasizing you are inside the store, not outside).

Here's an example from Cambridge Dictionary:

'Jane was a pharmacist in a chemist's in a small town'.

Verpassen Sie nicht die Gelegenheit, bequem von zu Hause aus eine Sprache zu lernen. Stöbern Sie in unserer Auswahl an erfahrenen Sprachlehrern und melden Sie sich jetzt zu Ihrer ersten Unterrichtsstunde an!