Omnicare Inc. is continuing its hostile takeover attempt of PharMerica Corp. by starting a tender offer to buy all outstanding shares of the pharmacy management services company for $15 each in cash.

Omnicare, based in Covington, Ky., went public last month with its $457 million offer after it said PharMerica, which is based in Louisville, Ky., refused to start negotiations about a possible deal. Omnicare, which dispenses drugs for nursing homes and other long-term care centers, has said PharMerica approached it about a possible combination of the companies in April.

Omnicare said Wednesday its "strong preference" is to negotiate a deal with the PharMerica board, but their refusal to do so has forced it to take the offer to shareholders.

"We are confident that PharMerica stockholders will recognize the significant value of our offer and send a clear message to PharMerica's Board that they should agree to negotiate a mutually acceptable transaction without further delay," Omnicare said in a statement.

PharMerica said in a separate statement its board will review the offer, and it urged shareholders to take no action. It said the offer is identical "in all material respects" to a previous one that PharMerica's board has already rejected.

PharMerica also said last month it had adopted a "poison pill" measure that will dilute the value of any shares Omnicare is able to buy.

Omnicare said Wednesday its tender offer expires Oct. 4 at midnight, EDT. It said the offer represents a 37 percent premium over PharMerica's closing stock price the last trading day before Omnicare announced its proposal.

Shares of Omnicare climbed 49 cents to $28.68 in Wednesday morning trading, while broader trading indexes rose more than 1 percent. PharMerica stock climbed 22 cents to $14.25.