The energy group retreated 1.4 percent overall despite higher global oil prices, with Cenovus down 5.1 percent to C$11.26 as the oil sands company also announced the departure of three senior executives.
The Toronto Stock Exchange's S&P/TSX composite index closes down 120.13 points, or 0.74 percent, at 16,016.46.
All 10 of the index's main sectors fell, with decliners outnumbering advancers by a ratio of 2.7-to-1 overall.
The financials group, which accounts for more than a third of the index's weight, lost 0.6 percent.
Drug company Valeant Pharmaceuticals International Inc fell 11.3 percent to C$25.11 after JPMorgan cut the stock to "underweight."
Bombardier Inc lost 1.9 percent to C$3.07 after the plane and train maker forecast 2018 revenue well short of analysts' estimates.
The largest percentage gainer was Mitel Network Corp , which rose 10.7 percent to C$10.46 after brokerage Craig-Hallum started coverage with a "buy" recommendation.