NEW YORK — Pfizer has clinched a blockbuster merger with a fellow drugmaker, one worth more than $150 billion, that can best be described in superlatives.

When it is announced — most likely Monday, people briefed on the matter said — the deal to buy Allergan, the maker of Botox, would be one of the biggest ever takeovers in the health care industry. And it would be the largest acquisition yet in a banner year for mergers.

Perhaps most important, it would be the biggest transaction aimed at helping a company shed its US corporate citizenship in an effort to lower its tax bill, in this case by billions of dollars. And it could become a flash point as the presidential race heats up.


A deal would come as the Obama administration is trying to crack down on these kinds of deals, known on Wall Street and in Washington as corporate inversions. Last week, the Treasury Department and the Internal Revenue Service revealed rules meant to further clamp down on the benefits of such mergers. The government has already lost billions of dollars in corporate tax revenue from inversions.

Pfizer has about 1,000 employees at a research center outside Kendall Square in Cambridge.

New rules introduced earlier this year deterred some companies determined to pursue inversions, including AbbVie, a drugmaker that called off its planned $54 billion takeover of Irish counterpart Shire. Still, Treasury officials said as recently as last week that only Congress can halt inversions.

Pfizer and Allergan are taking steps to sidestep the current rules altogether. Although Pfizer is significantly bigger, Allergan would technically be the buyer, according to people briefed on the matter.

Because Allergan has its headquarters in Dublin — even though the bulk of its operations are based in New Jersey — the planned transaction could avoid the Treasury rules.


But in most respects, Pfizer would lead the combined company, which would surpass Johnson & Johnson as the biggest drugmaker by revenue, with more than $60 billion in sales. Its product portfolio would run from Viagra, Celebrex, and pneumonia drugs to Botox and the cosmetic treatment Juvéderm. Analysts do not expect the merger to have much effect on the prices of the companies’ drugs.

Pfizer’s chief executive, Ian Read, would hold onto that role at the combined company, these people said. His counterpart at Allergan, Brent Saunders, is expected to take a top deputy role and a board seat.

The boards of both Pfizer and Allergan were scheduled to vote on Sunday to approve the transaction, the people briefed on the matter said. These people cautioned that final details were still being negotiated and that the talks might fall apart. News of the votes was reported earlier by The Wall Street Journal.

Representatives for Pfizer, Allergan, and the Treasury Department declined to comment.

Adopting Allergan’s home base of Ireland would yield significant savings for Pfizer, one of the oldest drugmakers in the United States. Its history runs from producing painkillers during the Civil War to penicillin in World War II. Pfizer’s tax rate last year was roughly 26.5 percent and is expected to be about 25 percent this year.

Its prospective merger partner, by contrast, reported a tax rate of just 4.8 percent for 2014, though its rate this year is about 15 percent.


President Obama last year declared that such moves were “unpatriotic.” But Read has long argued that an inversion is an important step in keeping the company competitive with foreign rivals based in lower-tax countries. Under the current rules, Pfizer must pay US corporate taxes on the billions of dollars in earnings from international operations if it ever tries to bring the money back to the United States. (The company kept $74 billion in earnings offshore last year to avoid that bill.)

He had already tried once to shift Pfizer’s home abroad, pursuing a $119 billion takeover bid for AstraZeneca of Britain. That campaign faltered amid fervent opposition from AstraZeneca and raised the hackles of lawmakers in the United States and Britain.

But Read, an accountant by training, has pressed ahead with his dream of a corporate inversion. Otherwise, he told The Wall Street Journal last month, Pfizer is fighting “with one hand tied behind our back.”

It was unclear whether the Obama administration would announce additional rules that would stymie the merger.

Under the terms of the proposed deal, Allergan shareholders would receive 11.3 Pfizer shares for each of their holdings, the people briefed on the matter said. That is worth about $363.63 a share, or 16 percent higher than Allergan’s closing price on Friday.

The transaction would also include a cash component, though one of the people described it as less than 10 percent of the deal’s overall value.

Pfizer shareholders would still own the majority of the combined business.


Robert Weisman of the Globe staff contributed to this report.