[mesmark]

"across the ..." means on the other side of ... or the path to the object

"It's across the street."
- means it's on the other side of the street.

Here the reference point is understood from the context or possibly the point of the speech act.

"across from ..." means opposite ...

"It's across from the supermarket."
- means 'it' and the supermarket sit directly opposite each other with the street between them

Here the reference point is the supermarket.

While they can both be used in some situations (but not interchangeably), the pharmacy example is pretty good to show the difference. with 'across the ...' you can have a much larger area between the reference point and the end point.

"He ran across the country."

Sorry, I'm rushed. I hope that makes sense.