Valeant fell 5.4 percent to C$216.73 after it said it was subpoenaed by U.S. prosecutors on its pricing, drug distribution and patient assistance programs.
The energy group slipped 0.6 percent, with oil falling for a fourth straight day as a U.S. crude buildup supported oversupply concerns.
"Canada is underperforming the U.S. today, likely due to a pullback in resources, which have been having a strong month so far in October," said Youssef Zohny, portfolio manager at StennerZohny Investment Partners+ of Richardson GMP Ltd.
The Toronto Stock Exchange's S&P/TSX composite index .GSPTSE ended down 46.36 points, or 0.33 percent, at 13,828.97.
It had risen steadily this month to above 14,000 by late last week, largely driven by renewed buying interest in the beaten-down energy sector.
"We've had a nice relief rally but definitely the risks are to the downside," Zohny said, citing Canada's exposure to a global economy showing signs of slowdown.
Advancing issues just outnumbered decliners, by 122 to 119, with most sectors posting only moderate gains or losses overall.
Reporting by Alastair Sharp; Editing by James Dalgleish and Matthew Lewis