The two companies plan to maintain their own brands after the deal is completed, McKesson said.
McKesson shares rose $7, or 4.9 percent, to $150.05 in morning trading in the U.S. Earlier the stock reached an all-time high of $155. Celesio shares gained 5.5 percent at 22.91 euros in Frankfurt.
Fitch Ratings said it will review McKesson's ratings for a possible downgrade. Fitch has an 'A-' rating on McKesson's credit. That rating is investment grade and four notches above "junk" status.
"The proposed Celesio deal is strategically sound, but its funding is likely to significantly increase McKesson's debt load," Fitch said. The firm said McKesson has a lot of leverage considering its current rating.
McKesson also reported its fiscal second-quarter results on Thursday. The company said its net income rose 1 percent, to $404 million, or $1.74 per share, from $401 million, or $1.67 per share. Excluding one-time charges, McKesson said it earned $2.27 per share. Revenue grew 11 percent, to $32.95 billion from $29.78 billion.
Analysts expected McKesson to report net income of $2.04 per share and $32.14 billion in revenue, according to FactSet. The company's second quarter ended Sept. 30.
The company said it now expects adjusted net income of $8.40 to $8.70 per share in fiscal 2014, up from its previous estimate of $8.05 to $8.35 per share.
Analysts had forecast $8.32 per share on average.