The drugmaker AbbVie said on Tuesday that it planned to buy Allergan, the maker of Botox, for about $63 billion, in one of the biggest mergers in the health care industry this year.
If completed, the deal would give AbbVie a potent source of popular treatments as it faces the loss of patent protection for Humira, a treatment for rheumatoid arthritis that is the world’s top-selling drug.
It is the second-biggest takeover in the pharmaceutical industry announced this year, after Bristol-Myers Squibb agreed to buy Celgene, a maker of anticancer drugs, for $74 billion.
Under the terms of the deal, AbbVie will pay 0.866 of its shares and $120.30 in cash for each share of Allergan. That is worth about $188.24 a share as of Monday’s closing prices, a nearly 45 percent premium on Allergan’s share price.
“This is a transformational transaction for both companies and achieves unique and complementary strategic objectives,” Richard A. Gonzalez, AbbVie’s chairman and chief executive, said in a statement. Mr. Gonzalez will remain as chief executive. Allergan’s chief executive, Brent Saunders, will join AbbVie’s board.
Mr. Saunders said that the proposed deal “creates compelling value” for his company’s shareholders and customers.
In buying Allergan, AbbVie is looking for a way to diversify beyond Humira, which — with sales of nearly $20 billion last year — accounted for nearly 60 percent of the company’s total revenue.
AbbVie has built Humira into a huge seller by expanding its approved uses and raising its price, but competing biosimilar versions are already available in Europe and they are scheduled to go on sale in the United States in 2023.
In a conference call Tuesday morning, Mr. Gonzalez said the acquisition “will have a profound impact on AbbVie’s overall growth story, while addressing concerns about the company’s reliance on Humira.”
Though Allergan sells a range of products, it, too, has struggled to diversify beyond its line of aesthetic medications and Botox, which has brought in billions of dollars with its expanded use beyond smoothing frown lines to treat migraine headaches and other health conditions.
In 2016, Allergan’s planned $152 billion merger with Pfizer fell apart after the Obama administration changed the tax rules to make it more difficult for American companies to lower their taxes by using mergers to shift their headquarters overseas. Since then, Allergan, an Irish company whose business is primarily in the United States, has been exploring either a sale or a split.
Shares in AbbVie, based near Chicago, were down 15 percent in morning trading on Tuesday, while Allergan shares were up about 27 percent.