At 10:06 a.m. EST (1506 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 19.76 points, or 0.12 percent, at 16,329.68.
Decliners outnumbered advancers by almost two to one.
Nine of the index's 10 main groups were in negative territory, with sharp gains for cannabis stocks helping the healthcare sector to rise.
Canopy Growth Corp rose 9.7 percent to $37.29, Aurora Cannabis was up 6.1 percent at $13.96, and Aphria Inc was up 9.7 percent at $21.02.
Cannabis producers had dipped last week, capping a recent rally, after the US Department of Justice rescinded a policy that had eased enforcement of US federal marijuana laws in states that had legalized the drug. Canada is working toward legalizing recreational use later this year.
Industrials fell 0.2 percent, with WestJet Airlines Inc falling 2.3 percent to C$25.44 after one of its planes was involved in an on-ground collision at a Toronto airport late on Friday. Larger rival Air Canada was off 1.9 percent at C$24.22.
The energy group and the materials group, which includes precious and base metals miners and fertilizer companies, each fell 0.3 percent.
Valeant Pharmaceuticals International Inc was down 3 percent at C$28.71.
The financials group slipped 0.1 percent.