---
title: "Pfizer → Allergan"
url: "https://mahouse.world/ma-deals/pfizer-buy-allergan-2015"
type: "M&A deal"
year: "2015"
---

# Pfizer → Allergan

A $160bn deal structured almost entirely for its tax address. Killed by a US Treasury memo issued twelve days before signing became irreversible.

Abandoned in April 2016, two days after the US Treasury tightened the rules on the tax inversions that underpinned it.

- **Acquirer:** Pfizer (United States)
- **Target:** Allergan (Ireland)
- **Value:** $160B
- **Announced:** November 2015
- **Outcome:** Terminated
- **Sector:** Pharma
- **Deal type:** merger

## Why it mattered

Pfizer's US statutory rate was 35%; Allergan's Irish rate was 12.5%. The transaction was engineered as a reverse merger — smaller Allergan the nominal buyer, larger Pfizer the target — precisely so the combined entity would inherit Ireland as its tax domicile. Estimated annual tax savings ran well over $1bn.

## The story

Announced November 2015, the transaction was flagged by the Obama administration as the standing example of everything wrong with the inversion loophole. On 4 April 2016 the US Treasury issued a new rule specifically targeting "serial inverters" — Allergan, which had itself been assembled through prior inversions, met the test almost by name. The economics collapsed inside 48 hours. Pfizer terminated on 6 April and paid Allergan the $150m break fee.

## What it left behind

The template for a regulator killing a deal by writing new rules mid-negotiation. Every large cross-border pharma deal signed since carries a "material change in law" clause that names the Pfizer–Allergan collapse as the reason it exists.

## At a glance

- **Structure:** Reverse merger, tax inversion (Allergan as nominal acquirer)
- **Announced:** 23 Nov 2015
- **Terminated:** 6 Apr 2016
- **Cause of death:** US Treasury & IRS rule (Notice 2016-16, targeting serial inversions)
- **Break fee paid:** $150M
- **Reimbursed expenses:** $400M+
- **Estimated annual tax saving lost:** $1B+