Exhibit 5.1

[LETTERHEAD OF SKADDEN, ARPS, SLATE, MEAGHER & FLOM LLP]

December 4, 2012

McKesson Corporation

One Post Street

San Francisco, California 94104

Ladies and Gentlemen:

We have acted as special counsel to McKesson Corporation, a Delaware corporation (the “Company”), in connection with the public offering of $500,000,000 aggregate principal amount of the Company’s 0.95% Notes due December 4, 2015 and $400,000,000 aggregate principal amount of the Company’s 2.70% Notes due December 15, 2022 (collectively, the “Securities”), issuable under the Indenture, dated as of December 4, 2012 (the “Indenture”) between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”). On November 29, 2012, the Company entered into an Underwriting Agreement (the “Underwriting Agreement”), with Merrill Lynch, Pierce, Fenner & Smith Incorporated and J.P. Morgan Securities LLC, as representatives of the several underwriters named therein (the “Underwriters”), relating to the sale by the Company to the Underwriters of the Securities.

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended (the “Securities Act”).

In rendering the opinions set forth herein, we have examined and relied on the following:

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We have also examined originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinions set forth below.

In our examination, we have assumed the genuineness of all signatures, including endorsements, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as facsimile, electronic, certified or photostatic copies, and the authenticity of the originals of such copies. As to any facts relevant to the opinions stated herein that we did not independently establish or verify, we have relied upon statements and representations of officers and other representatives of the Company and others and of public officials.

Our opinions set forth herein are limited to the Delaware corporate law and the laws of the State of New York that, in our experience, are applicable to transactions of the type covered by the Purchase Agreement, the Indenture and the Securities and, to the extent that judicial or regulatory orders or decrees or consents, approvals, licenses,

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authorizations, validations, filings, recordings or registrations with governmental authorities are relevant, to those required under such laws (all of the foregoing being referred to as “Opined on Law”). We do not express any opinion with respect to the law of any jurisdiction other than Opined on Law or as to the effect of any such non-opined-on law on the opinions herein which laws are subject to change with possible retroactive effect.

The Indenture and the Note Certificates are referred to herein collectively as the “Transaction Agreements.” “Organizational Documents” means those documents listed in paragraphs (v) and (vi) above.

Based upon the foregoing and subject to the limitations, qualifications, exceptions and assumptions set forth herein, we are of the opinion that the Securities have been duly authorized by all requisite corporate action on the part of the Company and duly executed by the Company under the DGCL, and when duly authenticated by the Trustee and issued and delivered by the Company against payment therefor in accordance with the terms of the Underwriting Agreement and the Indenture, the Securities will constitute valid and binding obligations of the Company, entitled to the benefits of the Indenture and enforceable against the Company in accordance with their terms under the laws of the State of New York.

The opinions stated herein are subject to the following qualifications:

(a) the opinions stated herein are limited by applicable bankruptcy, insolvency, reorganization, moratorium, fraudulent transfer, preference and other similar laws affecting creditors’ rights generally, and by general principles of equity (regardless of whether enforcement is sought in equity or at law);

(b) except to the extent expressly stated in the opinions contained herein, we do not express any opinion with respect to the effect on the opinions stated herein of (i) the compliance or non-compliance of any party to any of the Transaction Agreements with any laws, rules or regulations applicable to such party or (ii) the legal status or legal capacity of any such party to any of the Transaction Agreements;

(c) except to the extent expressly stated in the opinions contained herein, we have assumed that each of the Transaction Agreements constitutes the valid and binding obligation of each party to such Transaction Agreement, enforceable against such party in accordance with its terms;

(d) we do not express any opinion with respect to the enforceability of any rights to indemnification, contribution, exculpation, release or waiver that may be contrary to public policy or violative of federal or state securities laws, rules or regulations; and

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(e) to the extent that any opinion relates to the enforceability of the choice of New York law and choice of New York forum provisions contained in any Transaction Agreement, the opinions stated herein are rendered solely in reliance upon New York General Obligations Law sections 5-1401 and 5-1402 and Rule 327(b) of New York Civil Practice Law and Rules and are subject to the qualification that such enforceability may be subject to, in each case, the exceptions and limitations in such sections 5-1401 and 5-1402, as well as by principles of public policy, comity or constitutionality.

In addition, in rendering the foregoing opinions, we have assumed that neither the execution and delivery by the Company of the Transaction Agreements nor the consummation by the Company of the transactions contemplated thereby, including the issuance and sale of the Securities: (i) conflicts with or will conflict with the Organizational Documents, (ii) constitutes or will constitute a violation of, or a default under, any lease, indenture, instrument or other agreement to which the Company or its property is subject, (iii) contravenes or will contravene any order or decree of any governmental authority to which the Company or any of its property is subject, (iv) violates or will violate any law, rule or regulation to which the Company or its property is subject or (v) requires or will require the consent, approval, licensing or authorization of, or any filing, recording or registration with, any governmental authority under any law, rule or regulation of any jurisdiction.

We hereby consent to the filing of this opinion with the Commission as an exhibit to the Company’s Current Report on Form 8-K, being filed on the date hereof, and incorporated by reference into the Registration Statement. We hereby consent to the reference to our firm under the caption “Legal Matters” in the prospectus supplement dated November 29, 2012 and filed with the Commission. In giving this consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Securities Act or the Rules and Regulations. This opinion is expressed as of the date hereof unless otherwise expressly stated, and we disclaim any undertaking to advise you of any subsequent changes in the facts stated or assumed herein or of any subsequent changes in applicable law.