Federal regulators on Friday approved another alternative version of Remicade, an expensive injected drug widely used for rheumatoid arthritis and other immune system disorders.

The U.S. Food and Drug Administration approved Renflexis, developed by Samsung Bioepis Co. of South Korea. Its U.S. partner, Merck & Co. of Kenilworth, New Jersey, will market Renflexis.

Merck said Renflexis should be available in six months. The delay is required under U.S. regulations for a relatively new category called biosimilars. That's the industry term for generic but not quite identical versions of biotech dru...