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.
In the field of mergers and acquisitions, shareholder rights plans were devised in the early 1980s to prevent takeover bids by limiting a shareholder's right to negotiate a price for the sale of shares directly.
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.[1] The plan could be triggered, for instance, if any one shareholder buys 20% of the company's shares, at which point every other shareholder will have the right to buy a new issue of shares at a discount. If all other shareholders can buy more shares at a discount, such purchases would dilute the bidder's interest, and the bid cost would rise substantially. Knowing that such a plan could be activated, the bidder could be discouraged from taking over the corporation without the board's approval, and would first negotiate with the board to revoke the plan.[2]
The plan can be issued by the board of directors as an "option" or a "warrant" attached to existing shares, and it can only be revoked at the board's discretion.