Sources told CNBC that Walgreens Chief Executive Stefano Pessina and Amerisource Chief Executive Steven Collis met to discuss a potential tie-up, but those early-stage explorations ended without an agreement.
The sources, who cautioned the deal talks could once again resume, declined to be named because the information is confidential.
Shares of Walgreens fell nearly 3 percent on the news, while Amerisource's stock tumbled nearly 6 percent.
When The Wall Street Journal first reported earlier this month that Walgreens had made a takeover approach to Amerisource, the drug distributor's stock surged about 25 percent. The report had cautioned that a formal offer hadn't yet been made and that a deal might not materialize.
Walgreens already owns a roughly 26 percent stake in Amerisource.