Since the beginning of the year, marijuana companies have seen dramatic declines due to concerns regarding oversupply in Canada and the uncertainty of legalization in the U.S. Year to date as of June 28, the ETFMG Alternative Harvest ETF (NYSEMKT:MJ), an exchange-traded fund tracking the performance of pot stocks, has declined by more than 25%. During the same period, the S&P 500 index fell by less than 7%.