It's currently the Pharmaceutical Advertising Advisory Board that has to approve every ad that appears in a print medical journal. I have had discussions with them about ads that I had concerns about, ads that I thought were misleading and could potentially harm patients and in cases where there's a concern about inaccuracies that could cause harm, the PAAB usually refers those cases to Health Canada. Health Canada's general position when I have communicated with them has been that they don't proactively monitor every statement that is made and they don't have the resources to do that.
So I would say the short answer is that the pharmaceutical ads are not effectively being regulated right now. There really isn't a body that's looking carefully at the content of pharmaceutical ads to make sure that they are accurate and that patients are being protected.
Medical journals are also receiving revenue from the ads and they have an interest in displaying ads regardless of how accurate they are.