In a note, Koffler said she "can't see a reason to own this stock."
Endo's President and CEO Rajiv De Silva, in a press release, said the company also changed its forecast due to "greater than expected price erosion across the generics sector; and delays on regulatory actions related to certain Endo products."
Endo also announced a restructuring plan involving the layoff of 740 workers. It is reducing workers in Huntsville, Ala. and closing a faciilty in Charlotte, N.C.
Endo expects this restructuring to generate savings of roughly $15 million per quarter by the end of 2017.
But some analysts were unimpressed. "Weak longer term outlook, drastically lowered guidance, and pushed out de-levering targets (4.6x leverage) are the key reasons we view the stock as toxic, and downgrade to Underperform from Neutral," Koffler wrote.
Endo's forecast also weighed on other specialty drugmakers such as Horizon Pharma, Valeant and Mallinckrodt.
Endo shares closed down 39 percent at $16.17, just shy of its worst daily performance since June 2002 when it fell more than 46 percent, and its third worst day in the stocks history, back to 1996. Shares have lost more than half their value so far this year.
— CNBC's Chris Hayes contributed to this report.