Shares of Activision Blizzard Inc. extended their slide toward a 19-month low Thursday, after an analyst wrote that continued allegations of misconduct at the videogame company could further act as an “overhang” on the stock.

J.P. Morgan’s Alexia Quadrani cut her rating on Activision’s stock ATVI to neutral from overweight Thursday, writing that she was “reluctant to downgrade” a stock that’s been underperforming, but “recent negative headlines” about sexual-misconduct allegations “introduce a significant amount of uncertainty…