The completion follows clearance from regulatory authorities in various countries, allowing the two companies to begin combined operations on Wednesday.

The combined firm will have annual revenues of US$47 billion, behind U.S. rival Pfizer, which following its acquisition of Wyeth has annual sales of around US$75 billion.

The deal announced in March is the latest tie-up of big drugmakers facing increased cost pressures as key patents expire. Merck expects to achieve cost savings of approximately US$3.5 billion annually beyond 2011 as a result of the transaction.

The two firms already had a joint venture to market the cholesterol treaments Vytorin and Zeita.

Schering-Plough generates about 70 percent of its revenue outside of the United States, including more than US$2 billion in annual revenue from emerging markets.