Just days after rejecting a $456 million purchase offer, the board at PharMerica Corp. adopted a poison-pill defense. The measure is intended to prevent Omnicare Inc. from obtaining control of PharMerica “in a manner or on terms that are not in the best interests of the company,” according to the firm. The maneuver trades rights with PharMerica common stock. These rights would be exercisable should a person or group amass 15% or more of the company’s common stock without board approval or announces a tender offer that attempts to do so. The companies are the two largest providers of pharmaceutical services in the long-term care sector.