Poison Pill
Definition
A poison pill is a defense that lets existing shareholders buy new shares at a steep discount once a hostile bidder crosses an ownership threshold, massively diluting the bidder’s stake. Formally called a shareholder rights plan, it makes accumulating control prohibitively expensive without board approval.
In practice
The pill does not usually kill a deal; it forces the bidder to negotiate with the board instead of buying control in the open market. Courts have upheld pills as a legitimate defense, but once a company is clearly for sale, Revlon duties limit how a board can use them.