Pot stocks that have taken Wall Street by storm had their worst session since January 2016.

The biggest U.S. pot exchange-traded fund -- the ETFMG Alternative Harvest ETF -- fell more than 9 percent on higher-than-usual volume on Monday. The $750 million fund, ticker MJ, has lost one-fifth of its value in the last five sessions.

The Toronto-listed Horizons Marijuana Life Sciences ETF, with C$977 million ($746 million) in assets, fell a record 12 percent to the lowest level since August.

A number of closely followed stocks led the group to the downside, with Tilray Inc., valued around $11 billion, tumbling 16 percent and Aurora Cannabis Inc dropping 13 percent.

Tilray shares (TLRY) remain up more than 600 percent from its July public offering, while Aurora bulls will look to Tuesday’s debut on the New York Stock Exchange to halt its two-day drop.