Omnicare announced Monday that it is extending its offer to purchase all outstanding shares of rival PharMerica for $15 per share to Jan. 20. The original offer was scheduled to expire Dec. 2.

Support for the deal appears to be waning — in October, 47% of PharMerica shares were tendered in support of Omnicare’s offer. But since then, the number of shares tendered has dropped from 2.6 million to 13.8 million, the Associated Press reported.

PharMerica CEO Gregory S. Weishar said in a statement that the decline supports view that $15 per share is inadequate for PharMerica stockholders.

Omnicare, the largest institutional pharmacy provider in the long-term care sector, filed a lawsuit against PharMerica, the country’s second largest pharmacy provider, in an effort to persuade PharMerica to accept a $440 million purchase offer and stop a poison pill anti-takeover defense. Omnicare went public with a $715 million bid for PharMerica in August.