The information is not clear because CIC has contradicted itself.
This was taken from the CIC site (Page Updated in 2010) http://www.canadainternational.gc.ca/jamaica-jamaique/visas/medical-medicaux.aspx?view=d&lang=eng
Can I have a medical exam done in advance to speed up my application?
No. Unless you are being sponsored as a spouse, partner or dependent child, do not initiate any medical tests until you receive instructions and a personalized medical form from this office. Only original forms stamped by the visa office and indicating a valid file number will be accepted by Designated Medical Practitioners. We cannot issue medical instructions or forms until we have received your complete application.
Your examination results are only valid for 12 months from the date of your initial examination. If you are not admitted to Canada within this time, you will be required to undergo another0medical examination.
And this also came from CIC (Page updated in 2015) http://www.cic.gc.ca/english/information/medical/medexams-perm.asp
There are two ways you can have an immigration medical exam, depending on your immigration category
Wait for instructions once you have submitted your application
Undergo your exam before you submit your application (Required for certain categories: see your application guide)
This is called an up front medical exam.
You can contact a panel physician directly to get your medical exam.
Once your medical exam has been completed, the doctor will give you a document confirming that you underwent a medical exam. You must include a copy of that document with your application.
If the doctor works with the eMedical electronic system:
they will give you an up-front medical notification printout.
If the doctor works with a paper system:
they will give you a copy of the IMM 1017B Upfront Medical Report form.
You must attach that form to your application before you submit it to the visa office. If you apply online, you must upload that form before you can submit your application.