Allergan plc  (AGN - Get Report) shares surged the most in more than 25 years Tuesday after the Botox maker agreed to a $63 billion takeover from biopharmaceutical group AbbVie Inc.  (ABBV - Get Report) .

AbbVie will pay $188.24 each in cash and shares for Allergan's outstanding common stock, a 45% premium to the group's Monday closing price, in a deal that would value the Dublin-based group at around $63 billion. AbbVie said the deal will add around 10% to the group's adjusted earnings in the first year, and will be incorporated in Delaware upon completion.

AbbVie's Richard Gonzalez will continue as chairman and Chief executive officer, the company said, and its main headquarters will remain in North Chicago, Illinois. Allergan CEO Brent Saunders will join the combined group's board when the deal is closed, the companies said, which is expected to be in early 2020.

"This is a transformational transaction for both companies and achieves unique and complementary strategic objectives," Gonzalez said. "The combination of AbbVie and Allergan increases our ability to continue to deliver on our mission to patients and shareholders. With our enhanced growth platform to fuel industry-leading growth, this strategy allows us to diversify AbbVie's business while sustaining our focus on innovative science and the advancement of our industry-leading pipeline well into the future."

Allergan shares spiked 26.7% Tuesday, the biggest single-day advance in 26 years, to change hands at $164.00 each, a move that would flip the stock into a 25% gain for the year. AbbVie shares, meanwhile, slumped 14.77% to $66.83 each.

Allergan shareholders will receive $120.30 in cash, and 0.866 of an AbbVie share, for reach outstanding holding, the companies said, putting the total consideration at $188.24 per share. AbbVie also said it hopes that added free-cash flow from Allergan will support a debt reduction target of between $15 billion and $18 billion before the end of 2021.

Last month, Allergan posted stronger-than-expected first quarter earnings and boosted its full-year profit guidance as Botox sales drove top-line growth of 2%, to $3.6 billion, for the Dublin-based pharmaceutical group.

Looking into 2019, Allergan said it sees non-GAAP revenues in the range of $15.10 billion to $15.40 billion, around half a percent better than its prior forecast, with non-GAAP earnings of around $16.55, 20 cents ahead of the group's prior forecast.

Botox sales, Allergan said, rose 2% from last year to $648.2 billion, a good performance for the wrinkle treatment as the company prepares for a discounted competitor from rival Evolus Inc. (EOLS) .

The treatment, called Jeuveau, received FDA approval earlier this year after the agency first rejected the application citing deficiencies in its manufacturing and chemical composition. Evolus has said it plans to price the treatment at discount of between 20% and 25% to Allergan's Botox, which generated $3.5 billion in sales last year.

Evolus shares surged 4% to $14.25 each following the Allergan deal, a move that extends the stock's year-to-date gain to around 24%.

Shares of Botox Maker Allergan Get a Lift