Your medical expenses are reduced by 3% of your net income, and the balance is then claimed as a credit on your tax return. This means if your net income was greater than $26,667 then $800 of medical expenses will have no impact on your tax return. The maximum amount deducted from medical expenses changes each year, for 2017 it is $2268. This means if your income is greater than $75,600 then medical expenses totalling greater than $2268 will reduce your tax liability.
https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/rc4065/medical-expe...
Another possibility is if your income is low enough, you potentially already have a zero tax liability due to other credits. If your tax liability is at zero, it can not be reduced further.