Medical examinations are required when the period of invitation is greater than six months and the individual has resided, during the one year period immediately preceding the date of entry to Canada, for six consecutive months, in a designated country.
For a list of designated countries please see: http://www.cic.gc.ca/english/information/medical/dcl.asp.
If a medical examination is required, this can significantly add to the visa processing time. Depending on the individual's circumstances, the additional time required can be 1-3 months.
Medicals can be done with an approved immigration physicianand results submitted with the application for a work permit.