The underlying motivations for the 1908 legislation controlling opium importation and the legislation restricting sale of specific drugs in proprietary medicines came together in 1911. There are several precedents and I will not cover them here. Suffice it to say that the Opium Act was seen as inadequate for controlling opium, and the PPM was far from sufficient in controlling access to things like morphine and cocaine. Indeed, the prohibition on cocaine being included in any proprietary medicines (combined with the fact that several provinces had placed strict controls on the sale of cocaine in pharmacies) meant that it was already so heavily restricted that you needed to find a nefarious illegal vendor to get your hands on it.
This association with criminality, and the black market created by the restrictions of the two 1908 laws and the provincial pharmacy legislation, meant that people began to see the sale and use of drugs as itself a problem, not only because people would be breaking the law, but because they linked such drugs with a broader national decline. Indeed, when you make something illegal you increase its value because it is more difficult to get, and therefore can be purchased at a premium. Simple supply and demand. So criminals began to increase their trafficking activities, creating a feedback loop: things that were previously not illegal became associated with criminality, and thus became increasingly considered to represent social degradation.
These concerns were merged in the 1911 Opium and Drug Act, which placed heavier penalties and punishments on the sale of opium, morphine, and cocaine. It also included a schedule of drugs that were covered in the law’s restrictions, which mean that it would be pretty easy to add new drugs to the schedule. This was a concern when the law was debated in Parliament because some legislators thought that new drugs could be added through an “Order in council” which meant the government could add a drug to the schedule without such a change being debated in parliament. Although the government members argued that this would not happen, eventually it did.
All of these debates are preserved in the debates of the House of Commons and of the Senate. They used to be available at the Early Canadiana Online website but seem to have been removed when that site merged with another, becoming canadiana.ca. You might be able to find these through Hathi Trust because it seems to have a lot of stuff and is ever-expanding, but I’m not going to go looking for you.
The full publication of the legislation of 1911 is available at archive.org, beginning at p197.
Here is the law as passed.
Drug History Canada 