It's not already?
I live in US but it's been OTC here for... well as long as I can remember. Same as Frontline and Capstar. There are no restrictions on any of that stuff, just more involved ones like Revolution or Profender (dewormer).
I have also seen people able to buy Advantage Muliti online if they get it shipped in from Australia where it is sold as Advocate.
Out of curiosity now how much does Advantage cost in Canada?