(The Hill) — Twitter adopted a so-called poison pill Friday that would help prevent it from undergoing a hostile acquisition after Elon Musk put in an offer to buy the social media company for $43 billion.

The board unanimously adopted the limited duration shareholder rights plan. Under the plan, if any person or group acquires 15% or more of Twitter’s stock without the board’s approval, other shareholders are allowed to purchase additional shares at a discount.

“The Rights Plan will reduce the likelihood that any entity, person, or group gains control of Twitter through open market accumulation without paying all shareholders an appropriate control premium or without providing the Board sufficient time to make informed judgments and take actions that are in the best interests of shareholders,” Twitter said in a press release.

The plan is set to expire in one year. A spokesperson for Twitter said the company won’t be commenting further.

The move is Twitter’s counterattack to an offer Musk filed Wednesday with the Securities and Exchange Commission (SEC) to buy the company. Musk’s bid to take over Twitter came after he declined a spot on its board of directors, offered after he acquired a 9.2% stake in the company last week.

Dan Ives, an analyst at WedBush Securities, said the poison pill is a “predictable defensive measure” for the board, and it will “not be viewed positively by shareholders given the potential dilution and acquisition unfriendly move.” Via email, he said, “The Board has [their] back against the wall and Musk and shareholders will likely challenge the merits of the poison pill in the courts. We believe Musk and his team expected this poker move which will be perceived as a sign of weakness not strength by the Street.”

In Musk’s letter to Twitter board chair Bret Taylor, filed with the SEC, he said his offer was his “best and final.” It is not yet clear what Musk’s next move will be, but at a TED conference Thursday he said he has a Plan B that he was not yet sharing publicly.