"There's a lot of cost savings when you're dealing with acquisitions among pharmaceutical companies," said Juliane Keppler, managing director of the global tax and regulatory team within Nasdaq OMX Corporate Solutions. "It's less expensive to merge with a company than to go through all the R.& D.," she said, referring to research and development.
Because most big drug companies already have substantial international profits, inversions are particularly attractive. Once companies have headquarters overseas, they do not have to pay the relatively high United States tax rate on international earnings.
"For Pfizer, which has accumulated substantial profits in subsidiaries in low-tax foreign countries that would be taxed if paid to the U.S. parent, the territorial tax system is likely the most important tax benefit from such a merger," a report by the Congressional Research Service said.
Finally, the robust European pharmaceuticals industry means that potential acquirers have many targets.
Read MoreAllergan: Ignore Pershing Square
"There are more assets that fit and have some strategic rationale and are available because they are public," Mr. Faessen said. "In financial institutions, you have regulatory issues. In the consumer sector, you don't have as many public companies that are actionable."
Buoyed by the inversion trend, the health care sector has been one of the most active for deals this year, with an estimated $328.8 billion announced through July 10 — a 207 percent increase from the period a year earlier, according to Thomson Reuters.
That figure appears likely to rise sharply by the end of the year.
AbbVie hopes to reach a deal for Shire before July 18, when it must make a firm offer or walk away for up to six months under British takeover rules.
Under the proposal Shire's board has accepted, AbbVie would pay cash and stock equal to about 53.20 pounds, or $91 a share for Shire. Shire shareholders would own about 25 percent of the combined company.
The latest offer represents a particularly rich premium — 53 percent above Shire's closing price of £34.67 a share on May 2, the day before AbbVie's first approach.
AbbVie's former parent also participated in an inversion transaction on Monday.
Abbott, which spun out AbbVie last year, will sell its pharmaceuticals operations in Europe, Japan, Canada, Australia and New Zealand to Mylan, which will then invert.
Under the unusual structure, being called a spinversion, Abbott Laboratories will divest itself of assets to allow a different company to strike an inversion. Abbott said it did not intend to be a long-term shareholder in Mylan and would seek to sell its stake.
Mylan's chief executive, Heather Bresch, the daughter of Senator Joe Manchin, Democrat of West Virginia, had been searching for an inversion. The company unsuccessfully pursued the Swedish drug maker Meda this year, but will now move to the Netherlands with its acquisition of the Abbott assets.