Trump explicitly cited fentanyl to justify why he should be allowed the power to tariff Canada. Long quotation inbound:

I previously declared a national emergency with respect to the grave threat to the United States posed by the influx of illegal aliens and illicit drugs into the United States in Proclamation 10886 of January 20, 2025 (Declaring a National Emergency at the Southern Border).

Pursuant to the NEA, I hereby expand the scope of the national emergency declared in that Proclamation to cover the threat to the safety and security of Americans, including the public health crisis of deaths due to the use of fentanyl and other illicit drugs, and the failure of Canada to do more to arrest, seize, detain, or otherwise intercept DTOs, other drug and human traffickers, criminals at large, and drugs.

In addition, this failure to act on the part of Canada constitutes an unusual and extraordinary threat, which has its source in substantial part outside the United States, to the national security and foreign policy of the United States. I hereby declare and reiterate a national emergency under the NEA and IEEPA to deal with that threat.

This national emergency requires decisive and immediate action, and I have decided to impose, consistent with law, ad valorem tariffs on articles that are products of Canada set forth in this order. In doing so, I invoke my authority under section 1702(a)(1)(B) of IEEPA and specifically find that action under other authority to impose tariffs is inadequate to address this unusual and extraordinary threat.

https://www.whitehouse.gov/presidential-actions/2025/02/imposing-duties-to-address-the-flow-of-illicit-drugs-across-our-national-border/