Pfizer Inc. will buy Allergan Plc for $160 billion, the pharmaceutical sales company announced Monday.

The deal will not only form the largest drug making company but will also become the largest instance of a U.S. company moving overseas to save taxes, according to Reuters. Pfizer plans to move its headquarters to Ireland, decreasing employment in the U.S. Technically, Allergan, which is based in Dublin, will buy Pfizer to avoid restrictions, but the combined company will still be known as Pfizer and will be led by its current CEO, Ian Read.

The deal has been under discussion since October. As a result of these discussions, Allergan’s stock shares fell 3.4 percent and Pfizer’s 2.6 percent.

The acquisition will add Allergan brands such at Botox wrinkle treatment, Namenda Alzheimer’s drug and Restasis dry-eye medicine to Pfizer’s product offerings. It is expected to be finalized in the second half of 2016.

Prior to the announcement, Pfizer anticipated paying about 25 percent in corporate taxes in 2015, while Allergan only anticipated about 15 percent. Once combined, they estimate a tax rate of 17 to 18 percent in 2017.

Eighteen months ago, Pfizer had attempted to acquire AstraZeneca Plc, but AstraZeneca management and UK politicians opposed the proposal. The Allergan deal will be Pfizer’s fourth large acquisition in the past 15 years.

The merger is expected to increase earnings per share by 10 percent in 2019. Currently the deal values Allergan shares at $363.63, and the company has $8 billion in debt.