I think it might be To Each His Own, which is also the name of a song as you might know. So if you remember a song with that name in the movie, this might be what you're looking for. I looked it up and the IMDB profile didn't say anything about the drugstore, but the adoption and subsequent "stalking" on the part of the mother is in there. This movie has Olivia de Havilland as the mother, if she is not who you remember, it may be a pre- or re-make of the one you saw.
Here's the link, so you can have a look for yourself:
http://imdb.com/title/tt0039040/maindetails