The companies, based in Dublin and London, have emerged as acquisition targets for U.S. drugmakers that seek foreign headquarters in order to bypass the U.S.’s 35 percent corporate tax rate, the highest in the developed world. AbbVie Inc. said July 18 it would buy Shire for $55 billion, and AstraZeneca spurned a $117 billion offer from Pfizer Inc. on May 19.

President Barack Obama has said U.S. companies that seek mergers with foreign firms to circumvent taxes are “unpatriotic,” and the Treasury Department yesterday said it’s reviewing a “range of options” to discourage tax inversions.

Changes may include policies that “meaningfully reduce the tax benefits” of the deals, the Treasury said in a statement.

Shire’s American depositary receipts fell 3.6 percent to $236.90 in New York, the biggest drop single-day drop since May 2, 2013. Each receipt is equal to three regular shares.

AstraZeneca ADRs declined 2.1 percent to $70.91, the lowest price since May 19. Each receipt is equal to one regular share. AbbVie, based in North Chicago, Illinois, fell 1.4 percent to $52.05 and New York-based Pfizer declined less than 1 percent to $28.28.

Trading ‘Unwind’

“We’re seeing the inversion trade unwind,” Alex Arfaei, an analyst at BMO Capital Markets in Toronto, said. “Many inversion targets had run up as the pace of deals picked up recently.”

The AbbVie-Shire deal would be attractive even without its tax benefits, he said, reducing AbbVie’s dependence on its $11 billion arthritis drug, Humira, providing “operational synergies” and allowing the company to spend cash it has accumulated outside the U.S.

“There is more to this deal than a tax inversion,” Arfaei said.

About three weeks ago, Treasury Secretary Jacob Lew said officials at his department had scoured “obscure provisions” of tax law and determined the government couldn’t act against inversions without congressional action. He and Obama have pushed Congress to curtail the deals.

The Treasury didn’t say when any changes would be announced. Options include reclassifying companies’ debt as equity, limiting deductions against the U.S. income tax and thus limiting their ability to shift income out of the country.

The administration also might prevent companies from using offshore assets that haven’t been taxed by the U.S. to finance inversion deals. Congress has deadlocked on the issue and is in a recess until September. Action probably won’t happen in an election year when Republicans believe they can take control of the Senate.