Twitter has announced it is fighting back against Elon Musk’s hostile takeover attempt, adopting a “poisoned pill” strategy.

Elon Musk made an unsolicited bid to purchase Twitter for $54.20 a share. Since making the offer, Musk has indicated his desire to take Twitter private if the deal goes through, but Twitter’s board is not interested.

The company has announced it is enacting “a limited duration shareholder rights plan (the ‘Rights Plan’)” to fight Musk’s takeover attempt. Under the plan, if any one shareholder gains a 15% or greater stake in the company, more shares would become available for all other shareholders to purchase at a discounted price. This would dilute Musk’s stake in the company, and allow other shareholders to effectively counter Musk by making it prohibitively expensive for him to purchase more than 15% of the company.

“The Rights Plan is intended to enable all shareholders to realize the full value of their investment in Twitter,” the company writes. “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.”

While Elon Musk may be the richest man in the world, Twitter’s poisoned pill may make the takeover too expensive even for him.