Twitter has dropped a major roadblock in front of Elon Musk’s effort to take over the company, leaving investors to wonder about the mercurial Tesla CEO’s next move.

The social media company has adopted a “poison pill” defense that makes it difficult for Musk or any other investor to buy Twitter without the board of directors’ approval.Musk, who currently owns about 9% of the company, last week disclosed an offer of about $43 billion, or $54.20 per share.

Twitter’s next likely move is to formally reject Musk’s offer, although it could negotiate.

Musk has a number of options which also include talks with the board, sweetening his offer, or even triggering the poison pill, which experts say would be disastrous for the company.

In a regulatory filing on Monday, Twitter’s board said it approved the defensive move to protect the company from “coercive or otherwise unfair” takeover tactics.

The board is leaving open the possibility of negotiating with Musk or another suitor.

The filing says the shareholder rights agreement should not interfere with any merger or offer approved by the board.

Flashback: Twitter activates ‘poison pill’ to defend against Musk

The Associated Press contributed to this article.