 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 as a way for directors 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 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 if used "proportionately" in others, including delaware in the united states. the typical shareholder rights plan involves a scheme whereby shareholders will have the right to buy more shares at a discount if one shareholder buys a certain percentage of the company's shares. the plan could be triggered, for instance, when 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. the plan can be issued by the board as an "option" or a "warrant" attached to existing shares, and only be revoked at the discretion of the board of directors. a shareholder who can reach a 20% threshold will potentially be a takeover bidder. if every other shareholder will be able to buy more shares at a discount, such purchases will dilute the bidder's interest, and the cost of the bid will rise substantially. knowing that such a plan could be called on, the bidder could be disinclined to the takeover of the corporation without the board's approval, and will first negotiate with the board so that the plan is revoked.for a description of a standard rights plan, see wachtell, lipton, rosen & katz, the share purchase rights plan in ronald j. gilson & bernard s. black, the law and finance of corporate acquisitions (2d ed. supp. 1999) at 10-18. 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. ==history== 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.harvard business school, case study 9-496-037, page 5 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 [[espionag
Mergers_and_acquisitions Metaphors_referring_to_war_and_violence Political_terminology Shareholders