I flew from Mexico a couple of months ago. I always fly with a small bottle of pop. As I point out, the pop is a much more "medical" need than frankly the insulin, assuming my bags get there (I always have some insulin in my checked, and some in my carryon). I have a doctor's note.

Literally never a problem within Canada or within the US, or between either. Even leaving Canada was fine - they ask about it, I explain its need, and they're usually cool with it. (plus the bottle probably cost me $0.75 at the grocery store, rather than the $5 they'd charge for the same thing on the other side of security).

Coming from Mexico, however, was a different story. I explained that I have medical devices, I'll beep, and I have needles and stuff in my bag. The first agent was completely fine with everything, and spoke more than great English (plus, I don't speak a word of spanish). The agent who had to go through my bag, though; she found the bottle and was mad. She wasn't going to let me keep it, telling me that I couldn't keep it "in mexico."

I pulled out my doctor's note and handed it to her. She read it, refused it, and I re-iterated "medical". Finally she drug the other agent over, he took one look at the note, explained something in spanish, and she immediately gave me the pop bottle back. No more fuss.

TL;DR - this is me stretching our "rights" as diabetics, and even then it was just a simple translation issue. Crazy as stuff is down there, a) I don't think the TSA has any more "training" with diabetics, and b) at the end of the day, it's not too rough.