U.S. citizens living in Canada generally should not have to pay any NIIT

To help fund "Obamacare," the U.S. is imposing the Net Investment Income Tax on 3.8% of a U.S. person's net investment income in excess of specified thresholds.  However, U.S. citizens living in Canada generally should not be subject to the NIIT on the basis that (in Nightingale’s view) it should qualify as a social security tax.

If the NIIT is not a social security tax, there nonetheless is a good argument that the U.S. should allow a foreign tax credit to such individuals under the Canada- U.S. Treaty, as there is no evident intent for the NIIT legislation to override the Treaty.

Neal Armstrong.  Summary of Kevyn Nightingale, "The Net Investment Income Tax:  How it applies to U.S. Citizens Abroad", International Tax, No. 73, December 2013, p. 9 under Treaties – Art. 24.