A poison pill, or shareholder rights plan, is an antitakeover device used by companies to deter hostile takeovers.It works by issuing new stock to existing shareholders at a discounted rate, diluting the ownership stake of potential acquirers.There are different types of poison pills, including flip-in and flip-over provisions, as well as dead-hand provisions.While poison pills can offer companies time and leverage, they may also protect management at the expense of shareholders and lead to legal battles.