Drug firm Dr Reddy's Laboratories today said it has received a warning letter from US health regulator over inadequate quality control procedures at its three drug manufacturing units in India. <br/><br/>The letter was issued by the the United States Food and Drug Administration (USFDA) yesterday for the three manufacturing plants at Srikakulam, Miryalaguda and Duvvada, the company said in a filing to BSE. This action follows earlier inspections of these sites by the agency in November 2014, January and February 2015, respectively, it added.