Closing Bell: Pfizer Could Back Out of Allergan Deal; U.S. Stocks Fall

Worries over the health of the global economy pressured stocks, erasing the bulk of the S&P 500's 2016 gains. European stocks also had a rough day, suffering their worst performance in six weeks. Pfizer (PFE) - Get Report is reportedly contemplating backing out of its deal with Allergan (AGN) - Get Report after the U.S. Treasury announced new rules to target inversion deals designed to reduce tax rates. The drugmaker's official party line is that it is reviewing how the laws impact its $160 billion merger. Antitrust regulators could hit Halliburton (HAL) - Get Report with a lawsuit as soon as this week in an attempt to block its Baker Hughes (BHI) deal which would combine the second and third-largest oil services companies. TheStreet's Keris Lahiff reports from Wall Street.