Visit to the allergist

So, Friday, I had a 2+ hour visit to the allergist to see what exactly I'm allergic to. It turns out just dust mites and mold. No trees. No pollen. I don't think even animals, although that might not have been in the testing. This surprised me just a little bit. I believe there were 16 substances tested, so... shrug.
What has been throwing me particularly is the direction from my doctor to get 2 bottles of Nasonex a month and discard the bottle after 15 days. He says there may be liquid in there, but the medicine's gone. Uh..? Really?? That doesn't seem quite right. I have plans on asking the pharmacist before I fill the scrip, but if anyone has experience with this, let me know. It sounded like a load of hooey.