Updated from 2:59 p.m. EDT

A merger between Merck ( MRK) and Schering-Plough ( SGP) could get roiled by a dispute over rights to a drug made by Johnson & Johnson ( JNJ).

Schering has international marketing rights to Remicade, a drug used to treat rheumatoid arthritis. Those rights may be void, however, if control of the company changes hands, according to The Associated Press.

"With Schering-Plough and Merck stating that they believe the merger will not impact these marketing rights, we believe that J&J is likely to make the next move via a potential lawsuit (against the companies) to claim the international rights, or by negotiating a payment to reacquire them," said Caris and Co. analyst David Moskowitz, in a note to investors, according to the AP.

If J&J gets full rights to Remicade, he said, that could lower Schering's value and hinder the deal with Merck.

According to Bloomberg, Remicade generated $2.19 billion for Schering-Plough last year and was Johnson & Johnson's top-selling drug, with $3.75 billion in sales.

Ratings outfit Fitch has said that in light of the potential merger, it may upgrade Schering but downgrade Merck on concerns about credit and debt.

Schering shares were recently up 4.7% to $21.08. Merck shares were up 5.8% to $22.20. J&J shares were up 2.5% to $47.78.

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