You can claim medical expenses for any 12 month
period, as long as it ends in 2018. So if you have expense from 2017, you may
be able to use them on your 2018 return. For expenses incurred in previous
years, you would need to amend your tax returns from those years. Here's how
you can make changes to returns you have already filed:
https://turbotax.community.intuit.ca/replies/2647992