Canada's Valeant Pharmaceuticals International extended its unsolicited offer to buy US Botox maker Allergan on Friday until the end of the year.
The $53.5 billion offer had been due to expire at 5:00 pm (2100 GMT). Valeant said it was now keeping it in play until 5:00 pm on December 31.
In June Allergan rejected for a third time a takeover bid from Valeant, saying the increased offer still "substantially undervalues" the company.
Allergan's board of directors unanimously rejected the bid backed by activist Valeant shareholder William Ackman's Pershing Square investment group.
The board said the takeover proposal creates significant risks and uncertainties for Allergan stockholders, and is not in their  best interests.
On a buying spree in the past four years, the Canadian company first made an unsolicited cash-and-stock buyout offer for Allergan on April 22, valuing it at $45.6 billion.
It then raised the offer to about $50 billion on May 28, and seeing that rejected, increased it again to $53.5 billion on May 30.
Irving, California-based Allergan charged that "Valeant's unsustainable business model relies on serial acquisitions and cost reductions, as opposed to top-line revenue growth and operational excellence."