The rules would apply to that inversion and any transactions that close after Monday.

Shares in Allergan tumbled 21 percent in after-hours trading following the Treasury announcement.

“We are conducting a review of the U.S. Department of Treasury’s actions announced today,” Pfizer and Allergan said in a joint statement on Monday. “Prior to completing the review, we won’t speculate on any potential impact.”

The rules do not end there. The Treasury Department also took aim at another feature of these so-called corporate inversion transactions: complicated internal loans that effectively move profits of United States-based businesses overseas. This tactic, known as earnings stripping, involves the American subsidiary borrowing from the parent company and using the interest payments on the loans to offset earnings — a cost that is not reflected on financial statements but lowers the tax bill.

Monday’s rules classify this intra-company transaction as if it were stock-based instead of debt, eliminating the interest deduction for the American subsidiary. This change applies not just to inversions but to any foreign company that has acquired an American entity and used this technique to achieve lower taxes.

Yet even by the Treasury Department’s own admission, the latest rules will not be enough to completely halt the flow of companies seeking to renounce their American citizenship, just as the two previous rule changes did not. Such a move would be possible only with an overhaul of the tax rules by Congress, which few believe will happen soon.