Canada-based Valeant Pharmaceuticals declared a May 12 deadline for its $5.7 billion bid for Cephalon and said it would walk if more than 50% of Cephalon shareholders do not consent to the offer, according to a Reuters report.

Valeant gave its deadline in a letter to Cephalon shareholders and at the same time urged them to replace their board of directors with a slate proposed by Valeant.

If Cephalon elects the new slate of directors and opens its books for review, Valeant said it might still be open to raising its offer modestly.

"We expect the nominees to fully discharge their fiduciary obligations and, if the nominees elected constitute a majority of Cephalon's Board, we would be willing to keep our offer open for a reasonable period of time while the new Cephalon Board explores Cephalon's options."