Twitter on Friday announced a measure that it intends to delay or even block the purchase of the company by Tesla billionaire CEO (CEO) Elon Musk.

The move, known in the business world as the “toxic pill,” is intended to make it more difficult for Musk, who already owns 9%, to buy more than 15% of Twitter’s shares.

Twitter announced the decision a day after Musk, the world’s richest man, offered the social network $43 billion (€39.7 billion) and threatened to sell all of his shares in the company, if the public offer was rejected.

According to a statement issued by the company, the board of directors unanimously adopted a “rights plan”, which “aims for all shareholders to receive the full value of their Twitter investment.”

“A rights plan will reduce the likelihood that any entity, person or group will gain control of Twitter through an open market purchase without paying premiums to all shareholders due to shareholder control or without giving the Board of Directors sufficient time to make decisions about actions that defend the interests of shareholders,” Explains the text.

This action, which is considered legal and expires within one year, does not mean that the board of directors cannot accept or consider the proposal to acquire the company, and sets out the same information memorandum.

After receiving Musk’s offer on Thursday, Twitter has already said it needs to review the proposal and lift use of the so-called “toxic pill,” according to the Wall Street Journal.