US STOCKS-Decline in healthcare stocks checks gains by GM, tech
* Valeant sinks after Citron report, drags Allergan, Endo
* GM, Boeing rise after strong results
* Twitter falls after Morgan Stanley downgrade
* Ferrari up on debut
* Dow up 0.37 pct, S&P down 0.08 pct, Nasdaq down 0.39 pct (Updates to early afternoon)
By Abhiram Nandakumar
Oct 21 (Reuters) - Wall Street was little changed in early afternoon trading on Wednesday as gains by General Motors and technology stocks were limited by a slide in healthcare stocks.
Valeant Pharmaceutical's U.S.-listed shares lost a fourth of their value and fell to $106.00 after short-seller Citron Research released a critical report on the company.
Allergan fell 4.6 percent to $251.37 and Endo International slumped nearly 13 percent to $54.98, weighing the most on the S&P 500 and the Nasdaq composite indexes. Both companies have business models similar to that of Valeant. Continuación...