Medical expenses must be claimed in a 12 month period with the last month ending in the tax year that you are filing. For example, you can claim from December 2016 to December 2017. If you have expenses for 2016 such as January 2016 to December 2016 that you did not claim, you would need to file an adjustment request for the 2016 income tax return. Please see the following FAQ for instructions on how to do so:
https://turbotax.community.intuit.ca/replies/2647992