Starboard Value on Monday disclosed a 4.6% stake in Perrigo prgo , and said the Dublin-based drugmaker needs to make immediate improvements to turn around its sagging stock price.

Starboard delivered a scathing letter to Perrigo’s chief executive John Hendrickson and its board on Monday, criticizing their performance since the company spurned a $26 billion takeover bid from generic drug maker Mylan myl late last year.

The hedge fund said the drugmaker should consider hiring an investment bank for advice on the sale of non-core assets or broader strategic alternatives.

“Unfortunately, since that time, results have gone decidedly in the wrong direction, and management’s promises have been woefully unfulfilled,” Starboard said.

Perrigo did not immediately return a call seeking comment.

 

Starboard is among the most aggressive activist hedge funds. It obtained board seats at Internet company Yahoo earlier this year and forced its recent sale to Verizon vz . The New York-based fund run by Jeffrey Smith has launched more campaigns this year against companies than any other activist, according to Thomson Reuters data.

 

Starboard’s letter identifies Perrigo’s prescription pharmaceuticals business as one that would be attractive to other companies in the sector.

The letter goes on to criticize Perrigo’s management for spending $100 million to thwart Mylan’s bid, and for receiving bonuses for its successful defense, while the company’s stock has dropped 50% since the offer.

Hendrickson took over the CEO job earlier this year, when Joe Papa left the top role to go to Valeant Pharmaceutical International vrx .

Starboard indicated it will not be patient awaiting a more detailed plan for changes from the chief executive.

“Although we recognize you are new to the CEO role and hope you will have fresh ideas, we also know that you have been at Perrigo for approximately 27 years,” Smith said in the letter, noting that no new strategic plans have been laid out by the company since Hendrickson took over.