I believe we're already seeing such warnings on product labels. For example, the label on a product containing ginkgo biloba will warn users not to take this product with an anticoagulant or a drug. Labels already contain warnings. Of course, this is not true of all product labels. Last year, there was a case of St. John's-wort interacting with a drug and causing a pregnancy. Health Canada and the industry are still feeling their way on this. However, the warnings are going out on product labels.
On May 8th, 2008. See this statement in context.