Oxytrol has been on the market since 2003 as a treatment for overactive bladder in women. Merck gained the right to sell the over-the-counter version from Actavis Inc., which makes the original version.
Shares of Merck rose 6 cents to $43.06 in afternoon trading. Shares of Actavis fell $2.30, or 2.7 percent, to $84.13 after the company issued a disappointing 2013 forecast. Actavis changed its name from Watson Pharmaceuticals on Thursday.