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What is a 'Poison Pill'

A poison pill is a tactic utilized by companies to prevent or discourage hostile takeovers. A company targeted for a takeover uses a poison pill strategy to make shares of the company's stock unfavorable to the acquiring firm.


The term poison pill is the common colloquial expression referring to a specially designed shareholder rights plan.

There are two types of poison pills:

1. A “flip-in” permits shareholders, except for the acquirer, to purchase additional shares at a discount. This provides investors with instantaneous profits. Using this type of poison pill also dilutes shares held by the acquiring company, making the takeover attempt more expensive and more difficult.

2. A “flip-over” enables stockholders to purchase the acquirer’s shares after the merger at a discounted rate. For example, a shareholder may gain the right to buy the stock of its acquirer, in subsequent mergers, at a two-for-one rate.

History and Functionality of Poison Pill

In regard to mergers and acquisitions, poison pills were initially constructed in the early 1980s. They were devised as a way to stop bidding takeover companies from directly negotiating a price for the sale of shares with shareholders and instead force bidders to negotiate with the board of directors.

Shareholder rights plans are typically issued by the board of directors in the form of a warrant or an option attached to existing shares. These plans, or poison pills, can only be revoked by the board. Of the two types, the flip-in variety is the most common.

Poison Pill Example

Flip-in poison pills may hold an attached option that permits shareholders to buy additional discounted shares if any one shareholder buys more than a certain percentage, or more, of the company’s shares. For example, a flip-in poison pill plan is triggered when a shareholder buys 25% of the company’s shares. When it is triggered, every shareholder, excluding the holder who purchased 25%, is entitled to buy a new issue of shares at a discounted rate. The greater the number of shareholders who buy additional shares, the more diluted the bidder’s interest becomes and the higher the cost of the bid. If a bidder is aware such a plan could be activated, it may be inclined not to pursue a takeover without board approval.

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