U.S. pharmacy services provider PharMerica adopted a poison pill to prevent hostile takeovers, two days after Omnicare went public with its bid to acquire the company for $441 million, according to Reuters.

The stockholder rights plan -- commonly referred to as a poison pill -- gives the company the right to issue new shares if a shareholder acquires a 15% stake or more, thus diluting the holdings of the shareholder.

PharMerica rejected Omnicare's bid saying it undervalued the company and could run into regulatory hurdles.

Deutsche Bank Securities is acting as financial adviser for PharMerica.