In some countries, a drugstore is like a small house with a window or big door and you just stand on the street or pavement in front of the window/door to buy. Only staff is allowed to go into that drugstore. In this case, it looks like a counter which sells medicine. See the picture below.
However, there are some bigger drugstores. You can go into the stores. The stores also sell other things such as umbrellas, tissues, sunglasses etc. You can go to a counter in the store to buy medicine. See the picture below.
Oxford dictionary has this example "I bought these sunglasses at the drugstore."
From Longman Dictionary has this example "For a number of years, artificial sweeteners were mainly used by diabetics, who bought the products in drugstores."
Is it correct to say "I buy aspirin in/at the drugstore"?
Note: some people prefer to say "I bought it from the drugstore"