After a seven month-long power struggle, US Botox manufacturer Allergan has agreed to be bought by Ireland-based drugmaker Actavis for $66 billion or $219 per share.

This is more than $12 billion above the current value of a hostile bid launched last year by Canadian pharmaceutical producer Valeant in cooperation with activist investor Bill Ackman's Pershing Square Capital Management.

In their latest bid launched in early November, Valeant and Ackman's which together own nearly 10% stake of Allergan, had offered $53 billion.

The takeover of Allergan by Actavis would be the largest ever for the company and the third-largest health care deal in US history, according to Standard & Poor's Capital IQ. It also would be the largest acquisition in a year full of major M&A transactions, making the $14 billion Germany's Bayer paid for Merck & Co's over-the-counter portfolio look like peanuts.

The agreed price represents a premium of about 54% over Allergan's stock market value before Valeant and Pershing Square began their takeover effort and is more than double the company's share at this point in 2013.

After the latest Actavis offer was announced, Valeant CEO J. Michael Pearson, said his company's initial evaluation was that "Valeant cannot justify to its own shareholders paying a price of $219 or more per share for Allergan." In a recent letter to Allergan's board, Pearson had said the company could offer cash and stock worth up to $200 per share.

The Actavis-Allergan merger would capitalize on a mammoth inversion coup achieved by Actavis in 2013. Based until then in the US state of New Jersey, an agreement to buy Irish drug maker Warner Chilcott allowed it to relocate its headquarters abroad to save tax.

The move touched off a wave of similar transactions or attempted transactions before the US Treasury Department in September of this year passed closed the loophole. However, as the Actavis deal had already been clinched, it remained unaffected.

With about $23 billion in revenue expected in 2015, the combined Actavis and Allergan would be one of the 10 largest global drugmakers, Cost savings could total $1.8 billion annually, the companies said. Allergan's blockbuster product, Botox will meet up with a suite of Actavis products in such fields as women's health and dermatology.

A lawsuit brought by Allergan against Mr. Ackman and Valeant, contending that their bid was illegal, is still pending, but observers said it most likely would be withdrawn.