DETROIT (AP) — 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 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 states the shareholder rights agreement should not interfere with any merger or offer approved by the board.

Although he said his offer was “final,” Musk may have to raise his bid to satisfy other shareholders. A Saudi prince who is among Twitter’s major shareholders scoffed at the offer last week in a tweet. Al Waleed bin Talal said he didn’t believe $43 billion is close to Twitter’s value given its growth prospects. Twitter shares hit an all-time high of $77.63 in March 2021.

When he made his offer public, Musk provided no details on financing, but such a disclosure could improve his chances. He could raise money by borrowing billions using his stakes in Tesla and SpaceX as collateral, and he could bring in other investors.

The poison pill would give stockholders as of April 25 the right to buy one one-thousandth of a share of preferred stock for each common share they own, at a price of $210. The rights are triggered if any person or group of investors buys 15% or more of the company’s shares without board approval.

The preferred stock would have the same voting rights as a common share, according to the filing, which does not specifically mention Musk.

The poison pill essentially would spell the end of Twitter if Musk or another investor acquires 15% or more of the company, said James Cox, a professor of corporate and securities law at Duke University.

Shareholders who exercise the rights and buy preferred stock at $210 would get $420 in Twitter stock or assets, he said. That would be more than Twitter can afford to pay, and likely would send the company into receivership, Cox said.

Cox added that no investor has ever crossed the line to activate a poison pill.

If Musk triggered the poison pill, he risks wiping out much of the money he has invested in Twitter because his stake would be diluted, Columbia University law professor Eric Talley said.

Twitter’s board has information the average shareholder doesn’t, such as earnings or market growth projections, and whether there’s reason to believe the share value is artificially depressed, Talley said. The board, he said, could just hold out.

Shares of Twitter closed Monday up 7.5% at $48.45, still $5.75 shy of Musk’s offer. That’s a sign investors are skeptical of whether Musk can pull off the deal.

Musk began accumulating Twitter shares in late January, ending up with about 9%. Only Vanguard Group controls more shares. A lawsuit filed last week in New York federal court claims Musk illegally delayed disclosing his stake so he could buy more shares at lower prices.