The most influential movers on the index included some of its major energy pipeline companies.
TransCanada Corp fell 4.7 percent to C$57.72 and Enbridge Inc declined 2.9 percent to C$55.80, while the overall energy group fell 1.6 percent.
Crude oil prices dropped for a fourth day as US stockpile figures showed a much bigger-than-expected build in crude inventories. US crude prices were down 3.0 percent at $45.27 a barrel, Valeant Pharmaceuticals International Inc fell 5.9 percent to C$29.34.
The drugmaker said it is in talks with third parties to sell its Salix stomach-drug business and other assets.
The heavyweight financials sector fell 0.5 percent, while information technology declined 0.7 percent. At 10:59 a.m. EDT (1459 GMT), the Toronto Stock Exchange's S&P/TSX composite index fell 91.64 points, or 0.62 percent, to 14,686.68.
Eight of the index's 10 main groups were lower. The materials group, which includes precious and base metals miners and fertilizer companies, added 1.1 percent. Goldcorp Inc rose 3.6 percent to $21.54, while Barrick Gold Corp climbed 2.7 percent to C$25.32.
Gold rose 1.2 percent to reach a four-week high above $1,300 an ounce as uncertainty over the outcome of the US election on Nov. 8 burnished the appeal of precious metals as a haven from risk.
Markets also awaited direction on the timing of a US interest rate hike from a Federal Reserve monetary policy decision due later on Wednesday.
Canadian uranium producer Cameco Corp reported a better-than-expected quarterly profit, largely helped by a jump in uranium sales volumes. Its shares jumped 5.3 percent to C$10.48.