The Wall Street Journal reported that McKesson was in advanced negotiations to buy German rival Celesio AG for more than $5 billion. The report on the newspaper's website cited "people familiar with the matter."
A McKesson spokesman told The Associated Press in an email that his company does not comment on "market rumors or speculation."
The Journal reported that if McKesson proceeds with a bid, it may announce it as early as this month. It said the company has gained access to the books of Celesio, one of Europe's three largest pharmaceutical wholesale distributors.
But the paper also said a deal could still fall apart, with price and financing arrangements being sticking points.
Shares of San Francisco-based McKesson climbed 3.8 percent, or $4.91, to $134.54 in Tuesday afternoon trading, while the Standard & Poor's 500 index fell nearly 1 percent. The company's stock had already climbed 34 percent so far this year as of Monday.
McKesson shares have jumped several times in 2013, including a gain in July after it reported fiscal first-quarter earnings that beat expectations and raised both its quarterly dividend and net income forecast. McKesson also provides information technology services, and an analyst said after that earnings report that the company clearly was "executing well across multiple fronts."