Three major Indian drugmakers received warning letters from the US health care watchdog in the last two months, highlighting that the country’s copycat drug sector is still plagued by quality issues amid rising global concern over tainted drug supplies. Cadila Healthcare Ltd., Glenmark Pharmaceuticals Ltd. and Lupin Ltd. are the latest local medicine makers to be reprimanded by the US Food and Drug Administration, exchange filings show. A plant with a warning letter is barred from introducing new products in the US, stymieing the company’s ability to grow revenue in the world’s top pharmaceutical market. The latest regulatory crackdown shows US regulator’s stance is hardening toward lapses in quality control even as it seeks cheaper drugs to contain health care costs. Indian pharmaceutical factories collectively produce about 40 per cent of the American generic drug supply and have been faulted in the past by the FDA for everything from lax documentation to open toilet drains and malfunctioning equipment.