As biosimilar versions of a big-selling Johnson & Johnson medicine reach the U.S. market, the health care giant accused one manufacturer of striking a “surreptitious” deal that could arguably reduce competition. And the dispute casts a spotlight on the complicated, behind-the-scenes machinations of the burgeoning market for these copycat medicines.

In a lawsuit filed in a federal court in New Jersey on Monday, J&J argued that Samsung Bioepis improperly reached an agreement to supply an unnamed health care conglomerate with a private label biosimilar version of Stelara, which has been widely prescribed for years to treat various inflammatory skin and bowel conditions.

Samsung Bioepis is one of several companies that settled patent litigation with J&J and, as part of those deals, won the right to sell a biosimilar version of Stelara beginning this year.  After settling with J&J last fall, Samsung subsequently contracted with Sandoz to market the treatment, which is named Pyzchiva, in the U.S., Canada, and more than two dozen countries in Europe.