The pharmaceutical sector dropped on Wednesday on the back of fears about risks to mergers and acquisitions (M&A) activity, with Shire sinking sharply.Concerns that stricter rules on so-called 'tax inversion' in the US could scupper Abbvie's £32bn takeover of London-listed Shire dented the latter's share price by over 6% in afternoon trade.The White House said it was "reviewing a broad range of authorities for possible administrative action" in an effort to stop American companies from taking over foreign businesses to shift their tax domicile overseas.UK pharma group Astrazeneca, which had recently been the target of a failed £69bn bid from Pfizer, saw its share price sink 4.4% on Wednesday as hopes eased that the US giant could return later in the future with a higher offer. Pfizer was explicit earlier in the year that it was looking to acquire to Astrazeneca to lower its tax bill.Larger peer GlaxoSmithKline was down 1%, though the stock was lower after going ex-dividend.Pharma stocks were also reeling on Wednesday after the collapse of two multi billion-dollar deals in other sectors, raising fears about a cooling of the M&A market. 21st Century Fox announced plans to walk away from the near-$80bn takeover of Time Warner and Sprint shelved plans to buy T-Mobile US for $30bn."Sentiment was further hurt after deals worth more than $100bn were pulled off the table last night in the US, with analysts questioning the robustness of the recent M&A boom," said market strategist Daniel Sugarman from ETX Capital.Other pharma stocks were lower, including GW Pharmaceuticals, Sinclair IS Pharma and Hikma Pharmaceuticals.Top performing sectors so far todayMobile Telecommunications 4,462.17 +0.99%Electricity 9,438.08 +0.74%Gas, Water & Multiutilities 5,915.43 +0.02%Bottom performing sectors so far todayHealth Care Equipment & Services 5,717.04 -3.49%Pharmaceuticals & Biotechnology 12,419.04 -3.26%Automobiles & Parts 8,000.86 -2.12%Food & Drug Retailers 3,245.80 -2.11%Household Goods & Home Construction 11,302.57 -1.88%BC