Walgreen, the biggest U.S. retail drugstore chain, is considering decamping to Switzerland in a quest for bigger tax breaks, just two years after reaping a hefty package of Illinois tax credits in exchange for keeping corporate jobs in the state.

Such a move, through a maneuver called an inversion, would cost the U.S. treasury $4 billion in tax revenue over the next five years, according to a new report by Americans For Tax Fairness, a tax reform advocacy group. It also may prompt other U.S. retailers, which typically pay high tax rates compared with large multinationals like Apple and General Electric, to seek foreign acquisitions in order to dramatically lower their bills.

Americans For Tax Fairness calculated Walgreen's possible tax savings based on determinations by outside analysts, who figured the company could lower its rate to about 20 percent from the current 35 percent if it were to incorporate in Switzerland. (Reincorporating abroad would not necessarily mean substantive changes in where company personnel and operations are housed.)

Because Walgreen's bottom line is significantly bolstered by taxpayer-funded Medicare and Medicaid drug benefits, accounting for one-third of all pharmacy sales, relocating offshore would represent an especially egregious exploitation of the leaky U.S. tax code, Americans for Tax Fairness concludes in the new report.

"Our research shows that Walgreens relies heavily on the U.S. taxpayer for its profits, and that an inversion would deprive our country of significant resources while giving the company an unfair advantage over its competitors," the report says.

A Walgreen inversion may be possible next year, should shareholders approve the purchase of the the Swiss company Alliance Boots, Europe's largest pharmaceutical wholesaler and retailer. In 2012, Walgreen bought a 45 percent stake in the company. At a meeting in April in Paris, some Walgreen shareholders pushed for an inversion, touting the potential tax savings, according to the new report. This seemed to prompt a shift in tone from company executives, who had previously downplayed the possibility of an inversion.

"We’ve never been a proponent of paying more taxes than we have to," said Rick Hans, a Walgreens vice president, at a later conference, according to the new report.

On Wednesday, in response to the report, Walgreens issued a statement. "As we’ve said before, we continue to analyze a number of issues as we move toward the window for exercising the second step of our transaction with Alliance Boots, and we will do what is in the best long-term interest of our company and its shareholders," spokesman James Graham said in an email. Inversions are possible when U.S. companies incorporate in another country, so long as 20 percent of company stock is owned by a foreign entity. After the inversion, the original U.S. company becomes a subsidiary of the foreign parent company, yet the foreign company is controlled by the shareholders of the original U.S. corporation.

The tax savings of moving a corporate address abroad can be enormous. Companies are no longer on the hook for paying U.S. taxes on profits earned abroad, potentially a huge benefit for companies with big overseas sales. Walgreens, because its stores are located primarily in the U.S., would likely realize big tax savings in a different way: By shifting large amounts of debt from its foreign operation to its domestic operation in order to offset profit, said Frank Clemente, the executive director of Americans for Tax Justice.

Recently, the U.S. pharmaceutical giant Pfizer tried -- and failed -- to acquire the British drug company AstraZeneca, a deal at least partly motivated by tax savings that might be realized through an inversion.

Over the last decade, tax aversion has become a standard corporate business practice. A recent study found that Fortune 500 companies have created a whopping 7,827 offshore shell companies to stash nearly $2 trillion in places like Bermuda and the Cayman Islands in order to avoid paying U.S. taxes. One common way U.S. companies exploit such shell companies is by transferring patents or trademarks abroad, and then paying their subsidiary licensing fees for the right to use those patents, thus reducing domestic profit.

U.S. retailers, though, typically pay taxes at or close to the 35 percent corporate rate. That's because they book all or most of their revenue in the U.S. and have few options for making it seem as if that revenue was earned elsewhere.

Though it is likely that Walgreen will complete its purchase of Alliance Boots, it is difficult to evaluate the odds the company will attempt an inversion. The New York Times has reported that at least one shareholder, the CtW Investment Group, cited the risk of removal from the S&P 500 and other stock indices as an argument for why the company should remain headquartered in the U.S.

The CtW group owns less than 1 percent of Walgreen's shares.