A friend  from Vancouver was recently fined
200 dollars at US border because she had some foods in her westy that she
was not aware were not allowed. I bring nothing across to US but some
tinned food and oatmeal. It's your responsibility to know what you have on
board.
They are becoming more sticky in this regards and very intolerant of people
"forgetting" what they have aboard. The link below addresses what you can
bring into Canada. I have never been asked about my prescription meds but
they are in original container. In general coming into Canada is less
restrictive in the food area than returning to the US. Presently Canada
doesn't want uncooked chicken because of bird flu. See the list below.
Coming back to US if agent asks about food  indicate that you
have no meat no fruit and no vegetables (Make very sure you do not!!!)
Stating that right off the bat is big positive in their view. In one
statement you are showing
you know what you have on board and you don't have
any restricted foods. There is little reason to bring food
back from Canada anyway as it cheaper in the US.

http://www.inspection.gc.ca/food/information-for-consumers/travellers/what-can-i-bring-into-canada-/eng/1389648337546/1389648516990