At the end of the day today, Merck & Co. Inc. and Schering-Plough Corp. will be one.

Merck expects to complete its $41 billion acquisition of Schering-Plough Corp. after the stock markets close today.

The deal will create the world's second-largest drugmaker, behind Pfizer Inc. Patent expirations on such top-selling drugs as Merck's asthma drug Singulair helped drive the combination.

Merck, which is based in Whitehouse Station, N.J., employs about 12,000 people in Montgomery County.

Schering-Plough is based in Kenilworth, N.J. The companies will begin operating as a single firm under the Merck name tomorrow.

The acquisition is expected to result in about 16,000 job losses globally, about 15 percent of the combined Merck/Schering-Plough workforce.

In March, Merck chief executive Richard Clark, who will head the new company, said he expected a minimal number of those reductions to occur in Montgomery County. The company today said that it would not answer questions beyond what was in today's news release on the deal's close.

The completion comes after the U.S. Federal Trade Commission approved the deal Thursday. Regulators in Europe and elsewhere, and shareholders of both companies, had already approved it.

The two companies have been jointly selling the cholesterol drugs Vytorin and Zetia for years.

The deal adds allergy drugs Nasonex and Clarinex to Merck's Singulair and its diabetes pill Januvia. It also gives Merck animal and consumer health products, and a promising biotech division.

Contact staff writer Miriam Hill at 215-854-5520 or hillmb@phillynews.com.

This article contains information from the Associated Press.