Valeant Pharmaceuticals International Inc. Chief Executive Officer Mike Pearson will step down in a board and management shakeup after a series of controversies that culminated in a 61 percent drop in the shares last week.

The drugmaker has started a search to replace Pearson, who will stay CEO until a successor is found, according to a statement Monday. Bill Ackman, the billionaire investor whose Pershing Square Capital Management LP is one of the drugmaker’s biggest investors, will join the board. Valeant said it asked former Chief Financial Officer Howard Schiller, who took over for Pearson during a two-month medical leave, to resign from the board following “improper conduct.” Schiller, in a statement via his law firm, said he never engaged in any improper conduct and refused to step down as director.

The latest developments cap three chaotic weeks since Pearson’s return from his leave, raising even more questions about Valeant’s accounting practices. Since the end of February, Valeant has delayed filing of its annual report pending an internal investigation by an ad hoc committee, disclosed an Securities and Exchange Commission probe and said it would restate several quarters worth of results. The company, which has been under scrutiny for months for its business practices, said Monday it has discovered “one or more material weaknesses” in internal controls.