You may want to read the manual for foreign workers (FW1) http://www.cic.gc.ca/English//resources/manuals/fw/fw01-eng.pdf - it really depends on what kind of internship you are seeking.
5.17. Work without a work permit R186(p)—Health care students
Foreign students, registered at foreign educational institutions outside Canada, in fields such as
medicine, occupational and physical therapy, nursing and medical technology may do their clinical
clerkships or short-term practicums in Canada. Written permission from the body that regulates
the particular health field is required in order to ensure that Canadian health care students are
placed for clinical practice first. The primary purpose of the practicum must be to acquire training
in the related field; therefore these positions will often be unpaid and should be of no more than
four months’ duration.
Foreign students in residency, extern or fellowship positions in Canadian clinical settings are not
included in this exemption.
Note: Persons entering Canada to perform this type of work are required to pass an immigration
medical exam according to R30.(See section 9 of this manual, see also OP 11 section 11,
and OP 15 section 5).
