Poison pill

A shareholder rights plan, colloquially known as a "poison pill", is a type of defensive tactic used by a corporation's board of directors against a takeover. Typically, such a plan gives shareholders the right to buy more shares at a discount if one shareholder buys a certain percentage or more of the company's shares. The plan could be triggered, for instance, if any one shareholder buys 20% of the company's shares, at which point every shareholder (except the one who possesses 20%) will have the right to buy a new issue of shares at a discount. If every other shareholder is able to buy more shares at a discount, such purchases would dilute the bidder's interest, and the cost of the bid would rise substantially. Knowing that such a plan could be activated, the bidder could be disinclined to take over the corporation without the board's approval, and would first negotiate with the board in order to revoke the plan.

The plan can be issued by the board of directors as an "option" or a "warrant" attached to existing shares, and only be revoked at the discretion of the board.

In the field of mergers and acquisitions, shareholder rights plans were devised in the early 1980s as a way to prevent takeover bidders from negotiating a price for sale of shares directly with shareholders, and instead forcing the bidder to negotiate with the board.

Shareholder rights plans, or poison pills, are controversial because they hinder an active market for corporate control. Further, giving directors the power to deter takeovers puts directors in a position to enrich themselves, as they may effectively ask to be compensated for the price of consenting to a takeover. Shareholder rights plans are unlawful without shareholder approval in many jurisdictions such as the United Kingdom, frowned upon in others such as throughout the European Union, and lawful only if used "proportionately" in others, including Delaware in the United States.

The poison pill was invented by mergers and acquisitions lawyer Martin Lipton of Wachtell, Lipton, Rosen & Katz in 1982, as a response to tender-based hostile takeovers. Poison pills became popular during the early 1980s in response to the wave of takeovers by corporate raiders such as Carl Icahn. The term "poison pill" derives its original meaning from a poison pill physically carried by various spies throughout history, a pill which was taken by the spies when they were discovered to eliminate the possibility of being interrogated by an enemy.

...
Wikipedia