well, im just not familiar with the laws in Canada, so I don't want to mislead you.
Appearantly, canadian laws and procedures are more relaxed than the their U.S. counterparts. However, you need to look at your local and federal laws to determine what are the requirements that a prescription must met in order to be considered legal.
In the U.S. for instance, medical prescriptions must be issued by a PH or Doctor who is allowed to practice medicine in the jurisdiction where the pres. has been issued. After a short interpretation of the law, such statute excludes any medical prescription issued overseas regardeless of the citzenship of the doctor who prescribed it. Nevertheless, people who travel from Mexico to the U.S. oftenly do so with their personal supply of meds with them, with no prescription in hand. Such is the case of people who's been diagnosed with hypertension for example. Evidently, there is some sort of discretion when enforcing the law. Common sense tells me that such discretion doesn't apply when a musclehead traveling with "roids" in hand gets caught at any checkpoint.
I imagine that a mexican script in Canada is no good, because once you show your mexican script for any reason, there is no way to confirm the legitimacy of such script. But this is just my guess.
In mexico for instance, scripts issued overseas are legit and legally valid, since the mexican law only requires that script had been issued by a Doctor. It doesn't clarify whether the doctor needs to be mexican, nor only doctors with a mexican license to practice medicine within mexican territory are the only ones who can. It simply says "by a doctor". Therefore, in such context, anyone who has a "Doctorate" degree, even if it's not medicine related can issue a script, since in mexico everyone who holds a doctorate degree are called doctores which means doctors.
Scarry isnt it?