In order to get FDA approval, the drugs have to manufactured under very stringent rules. The rules are the same whether the drugs are sold in the United States or elsewhere. The are additional FDA rules for the distribution and sale of the drugs. Once the drugs leave the plant for sale in Canada, they are out of FDA jurisdiction and that is why they are no longer FDA approved. But the drugs are manufactured the same, regardless where they are sold. It is entirely possible that governement rules for the distribution and sale of drugs in some countries are more stringent than they are in the United States. Theoretically, then, American drugs could be "better" in other countries but they wouldn't be FDA approved.

The main reason why Americans are prohibited from buying American drugs cheaper in Canada is because the American drug companies don't want us to.