In what’s called a “reverse-inversion,” Allergan, a small Dublin-based drug company that makes products such as Botox, will technically buy the US-based pharmaceutical behemoth Pfizer, which makes products such as Viagra and Lipitor.

The $160 billion merger, officially announced Monday, will allow Pfizer to move its executive offices to Ireland, thus lowering its tax rate, while also morphing into the world’s largest drug maker.

Such inversions, which are said to cost the American government billions in lost tax revenue, have drawn scorn from the Obama Administration and the Treasury Department. Last year, President Obama referred to the deals as “unpatriotic” loopholes and proposed to close them. And last week, the Treasury announced new rules to make such deals more difficult.

But Pfizer’s reverse-inversion skirts the rules, in part by keeping ownership split somewhat evenly between the two companies. After the deal is complete, current shareholders of Allergan, which has the majority of its operations in the US, will own 44 percent of the mega company. The remaining 56 percent will be owned by current Pfizer shareholders.

In Monday’s announcement, Pfizer said that the deal will deliver $2 billion in cost savings per year for the first three years. By 2018, the combined company, which will also be called Pfizer, will have an operating cash flow of $25 billion.

The merged company will also enjoy a 17 to 18 percent tax rate, Pfizer said. That’s down from Pfizer’s current US tax rate of about 25 percent.

Chairman and CEO of Pfizer Ian Read was quoted in the announcement as saying, “The proposed combination of Pfizer and Allergan will create a leading global pharmaceutical company with the strength to research, discover, and deliver more medicines and therapies to more people around the world.”

The company said it expects that the deal, which still requires shareholder and regulatory approval, will be done by the second half of 2016. Lawmakers and healthcare professionals are currently monitoring and investigating such large mergers between pharmaceutical companies and their impacts on healthcare costs.