I take a lot of different prescription drugs, which require renewal at different times, or may require new prescriptions. One of these is a Class III controlled substance. How do I get new prescriptions or refills in Canada, or should I try to make sure I have all I require before I leave? My US pharmacy may not be willing to refill early, especially the controlled substance.
You likely won't be able to fill the prescriptions in Canada without visiting a Canadian doctor (which will be at your cost, as you're not covered by our public health system) and obtaining new prescriptions. Mind, some drugs are over-the-counter here and are by prescription in the U.S. (e.g. 222s, which are ASA + codeine).
Ensure you not only have the medications in their original packaging (as received from your pharmacy, that is), but that you have a copy of your doctor's prescription with you as well. The border agents have the right to inspect that paperwork to determine that the prescriptions are indeed yours.
Usually, if you will be traveling out of the country, you can renew prescriptions early (I have done this myself in the past). You may need to show your travel plans. Talk to your pharmacist sooner rather than later.
I'm not from the US, but Europe, and in a similar situation (several prescription and other meds, including some that may be problematic in some countries).
Just get your doctor and/or pharmacy to get you a refill for the duration of your trip, and for the controlled substances a letter stating that yes, you really were prescribed those and have them for personal use only.
Both doctors, pharmacists, and border agents deal with such situations on a regular basis so it should not be something they're not used to for any of them.
Where you'd have to be careful is when taking OTC meds like Aleve or Voltaren, which might actually be illegal to own without a prescription in some countries (I know Voltaren tablets are prescription only in Canada, Voltaren cream is not, go figure). Last year a European tourist was arrested and thrown in prison in Egypt for trying to bring a few boxes of Aleve across the border for a sick family member in an Egyptian hotel. They were considered illegal drugs and the traveler arrested for trying to enter the country with controlled substances without a license. I don't think you'll get into that kind of trouble on your trip as long as you have proper paperwork showing that yes, they're your drugs and on a prescription, but with freely sold stuff from the supermarket it can happen.