Exhibit 5.1

February 9, 2009

McKesson Corporation

McKesson Plaza, One Post Street

San Francisco, California 94104

Re: Registration Statement on Form S-3 of McKesson Corporation

Ladies and Gentlemen:

We have acted as special counsel to McKesson Corporation, a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-3 (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”). The Registration Statement relates to, among other things, the issuance and sale from time to time by the Company, pursuant to Rule 415 of the General Rules and Regulations promulgated under the Act, of the following securities of the Company: (i) shares of common stock, par value $0.01 per share, of the Company (the “Common Stock”); (ii) shares of preferred stock, par value $0.01 per share, of the Company (the “Preferred Stock”), in one or more series; (iii) fractional shares of the Company’s Preferred Stock represented by depositary shares (the “Depositary Shares”) evidenced by depositary receipts (the “Receipts”), which may be issued pursuant to one or more deposit agreements (each, a “Deposit Agreement”), proposed to be entered into between the Company and a bank or trust company to be named (the “Depositary”); (iv) senior debt securities, senior subordinated debt securities, subordinated debt securities and junior subordinated debt securities (collectively, the “Debt Securities”), which may be issued under the Indenture (the “Indenture”), dated March 5, 2007, by and between the Company and The Bank of New York Mellon Trust Company, N.A. (formerly known as The Bank of New York Trust Company, N.A.), as trustee (the “Trustee”), which has been filed as an exhibit to the Registration Statement; (v) warrants (“Warrants”) to purchase Preferred Stock, Common Stock or Debt Securities (the “Warrants”) as shall be designated by the Company at the time of the offering and issued pursuant to one or more warrant agreements (each, a “Warrant Agreement”) proposed to be entered into between the Company and warrant agents to be named (the “Warrant Agent” or “Warrant Agents”); (vi) stock purchase contracts of the Company (“Stock Purchase Contracts”) obligating the holders to purchase from or sell to the Company, and the Company to sell to or purchase from the holders, Common Stock at a future date or dates, which may be issued under one or more purchase contract agreements (each, a “Stock Purchase Contract Agreement”) proposed to be entered into by the Company and a purchase

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contract agent or agents to be named (the “Stock Purchase Contract Agent” or “Stock Purchase Contract Agents”); and (vii) stock purchase units of the Company (“Stock Purchase Units”), each representing ownership of a Stock Purchase Contract and any of the Debt Securities or debt obligations of third parties, including United States Treasury securities, in each case securing a holder’s obligation to purchase shares of Common Stock under the Stock Purchase Contract. The Common Stock, the Preferred Stock, the Depositary Shares, the Debt Securities, the Warrants, the Stock Purchase Contracts and the Stock Purchase Units are collectively referred to herein as the “Offered Securities.”

This opinion is being delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Act.

In connection with this opinion, we have examined originals or copies, certified or otherwise identified to our satisfaction, of:

(i) the Registration Statement;

(ii) the Indenture, filed as an exhibit to the Registration Statement;

(iii) the forms of underwriting agreements, filed as exhibits to the Registration Statement to be entered into by the Company and one or more underwriters to be named in connection with any underwritten offering of equity or debt securities;

(iv) the forms of Warrant Agreements, filed as exhibits to the Registration Statement;

(v) the form of Deposit Agreement, filed as an exhibit to the Registration Statement;

(vi) the Amended and Restated Certificate of Incorporation of the Company, as amended to the date hereof (the “Certificate of Incorporation”);

(vii) the Amended and Restated By-Laws of the Company, as currently in effect (the “By-Laws”); and

(viii) certain resolutions adopted on May 26, 2004, January 26, 2005, October 26, 2007 and January 21, 2009 by the Board of Directors of the Company (the “Board of Directors”) relating to the registration of the Offered Securities.

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

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In our examination, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed, photostatic, electronic or facsimile copies and the authenticity of the originals of latter documents. As to any facts material to the opinions expressed 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. In making our examination of executed documents or documents to be executed, we have assumed that the parties thereto, other than the Company, had or will have the power, corporate or otherwise, to enter into and perform all obligations thereunder and have also assumed the due authorization by all requisite action, corporate or other, and the execution and delivery by such parties of such documents and, as to parties other than the Company, the validity and binding effect on such parties. We have also assumed (a) that each of the Deposit Agreement and the Warrant Agreements will be duly authorized, executed and delivered by the Depositaries and the Warrant Agents, as the case may be, in substantially the forms reviewed by us, (b) that the Indenture was duly authorized, executed and delivered by the Trustee, and (c) that any Depositary Shares, Warrants or Debt Securities that may be issued will be manually signed or countersigned, as the case may be, by duly authorized officers of the Depositaries, the Warrant Agents or the Trustee, as the case may be. In addition, we have assumed that the terms of the Offered Securities will have been established so as not to, and that the execution and delivery by the Company of, and the performance of its obligations under, the Offered Securities, the Deposit Agreements, the Indenture, the Deposit Agreements, the Warrant Agreements and the Purchase Contract Agreements, will not violate, conflict with or constitute a default under (i) any agreement or instrument to which the Company is subject, (ii) any law, rule or regulation to which the Company is subject, (iii) any judicial or regulatory order or decree of any governmental authority, or (iv) any consent, approval, license, authorization or validation of, or filing, recording or registration with any governmental authority.

Our opinions set forth below are limited to the General Corporation Law of the State of Delaware (“DGCL”) and those laws of the State of New York that, in our experience, are normally applicable to transactions of the type contemplated by the Registration Statement and to the extent that judicial or regulatory orders or decrees or consents, approvals, licenses, 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 stated. The Offered Securities may be issued from time to time on a delayed or continuous basis, and this opinion is limited to the laws, including the rules and regulations, as in effect on the date hereof, which laws are subject to change with possible retroactive effect.

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Based upon the foregoing and subject to the limitations, qualifications, exceptions and assumptions set forth herein, we are of the opinion that:

1. With respect to any offering of Common Stock (the “Offered Common Stock”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Common Stock has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Common Stock is to be sold pursuant to a firm commitment underwritten offering, the underwriting agreement with respect to the Offered Common Stock has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance of the Offered Common Stock and related matters; (v) the terms of the issuance and sale of the Offered Common Stock have been duly established in conformity with the Certificate of Incorporation and the By-Laws so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company; and (vi) certificates in the form required under the DGCL representing the shares of Offered Common Stock are duly executed, countersigned, registered and delivered upon payment of the agreed upon consideration therefor, the shares of Offered Common Stock (including any Common Stock duly issued upon conversion, exchange or exercise of any Preferred Stock, Debt Securities or Warrants), when issued and sold in accordance with the applicable underwriting agreement with respect to the Offered Common Stock or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be duly authorized, validly issued, fully paid and nonassessable, provided that the consideration therefor is not less than $0.01 per share of Common Stock.

2. With respect to the shares of any series of Preferred Stock (the “Offered Preferred Stock”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the shares of the Offered Preferred Stock has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Preferred Stock is to be sold pursuant to a firm commitment underwritten offering, the underwriting

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agreement with respect to the shares of the Offered Preferred Stock has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the shares of the Offered Preferred Stock and related matters, including the adoption of a Certificate of Designation for the Offered Preferred Stock in accordance with the applicable provisions of the DGCL (the “Certificate of Designation”); (v) the filing of the Certificate of Designation with the Secretary of State of the State of Delaware has duly occurred; (vi) the terms of the Offered Preferred Stock and of their issuance and sale have been duly established in conformity with the Certificate of Incorporation, including the Certificate of Designation relating to the Offered Preferred Stock, and the By-Laws of the Company so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company; and (vii) certificates in the form required under the DGCL representing the shares of the Offered Preferred Stock are duly executed, countersigned, registered and delivered upon payment of the agreed-upon consideration therefor, the shares of the Offered Preferred Stock (including any Preferred Stock duly issued upon conversion, exchange or exercise of any Preferred Stock, Depositary Shares, Debt Securities or Warrants), when issued or sold in accordance with the applicable underwriting agreement or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be duly authorized, validly issued, fully paid and nonassessable, provided that the consideration therefor is not less than $0.01 per share of Preferred Stock.

3. With respect to any Depositary Shares (the “Offered Depositary Shares”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Depositary Shares has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Depositary Shares are to be sold pursuant to a firm commitment underwritten offering, an underwriting agreement with respect to the Offered Depositary Shares has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the Offered Depositary Shares and related matters; (v) the terms of the Offered Depositary Shares and of their issuance and sale have been duly established in conformity with the applicable Deposit Agreement so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or

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instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company and the applicable Depositary; (vi) the Offered Depositary Shares have been duly executed, delivered and countersigned in accordance with the provisions of the applicable Deposit Agreement and duly issued and sold in the applicable form filed as an exhibit to the Registration Statement in the manner contemplated in the Registration Statement or any prospectus supplement or term sheet relating thereto; (vii) the Preferred Stock relating to such Offered Depositary Shares have been duly issued and paid for in the manner contemplated in the Registration Statement and any prospectus supplement relating thereto; and (viii) the Receipts evidencing such Offered Depositary Shares are duly issued against the deposit of such Preferred Stock in accordance with the Deposit Agreement, such Receipts will be validly issued and will entitle the holders thereof to the rights specified therein and in the Deposit Agreement, the Offered Depositary Shares, when issued and sold in accordance with the applicable Deposit Agreement and the applicable purchase agreement or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, except to the extent that enforcement thereof may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or other similar laws now or hereafter in effect relating to creditors’ rights generally, (b) general principles of equity (regardless of whether enforceability is considered in a proceeding at law or in equity), and (c) public policy considerations which may limit the rights of parties to obtain remedies.

4. With respect to any series of Debt Securities (the “Offered Debt Securities”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act and the Indenture has been qualified under the Trust Indenture Act of 1939, as amended; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Debt Securities has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Debt Securities are to be sold pursuant to a firm commitment underwritten offering, the underwriting agreement with respect to the Offered Debt Securities has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the Offered Debt Securities and related matters; (v) the terms of the Offered Debt Securities and of their issuance and sale have been duly established in conformity with the applicable Indenture so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company so as to comply with any requirement or

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restriction imposed by any court or governmental body having jurisdiction over the Company; and (vi) the Offered Debt Securities have been duly executed and authenticated in accordance with the provisions of the Indenture and duly delivered to the purchasers thereof upon payment of the agreed-upon consideration therefor, the Offered Debt Securities (including any Debt Securities duly issued upon conversion, exchange or exercise of any Preferred Stock, Debt Securities or Warrants), when issued and sold in accordance with the Indenture and the applicable underwriting agreement, if any, or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, except to the extent that enforcement thereof may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or other similar laws now or hereafter in effect relating to creditors’ rights generally, (b) general principles of equity (regardless of whether enforceability is considered in a proceeding at law or in equity), (c) public policy considerations which may limit the rights of parties to obtain remedies, (d) the waivers of any usury defense contained in the Indenture or Offered Debt Securities which may be unenforceable, (e) requirements that a claim with respect to any Offered Debt Securities denominated in a currency, currency unit or composite currency other than United States dollars (or a judgment denominated other than in United States dollars in respect of such claim) be converted into United States dollars at a rate of exchange prevailing on a date determined pursuant to applicable law, and (f) governmental authority to limit, delay or prohibit the making of payments outside the United States or in foreign currencies, currency units or composite currencies.

5. With respect to any series of Warrants (the “Offered Warrants”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Warrants has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Warrants are to be sold pursuant to a firm commitment underwritten offering, an underwriting agreement with respect to the Offered Warrants has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the Offered Warrants and related matters; (v) the terms of the Offered Warrants and of their issuance and sale have been duly established in conformity with the applicable Warrant Agreement so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company and the applicable Warrant Agent; and (vi) the Offered Warrants have been duly executed, delivered and

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countersigned in accordance with the provisions of the applicable Warrant Agreement and duly issued and sold in the applicable form filed as an exhibit to the Registration Statement and in the manner contemplated in the Registration Statement or any prospectus supplement or term sheet relating thereto, the Offered Warrants, when issued and sold in accordance with the applicable Warrant Agreement and the applicable underwriting agreement or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, except to the extent that enforcement thereof may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or other similar laws now or hereafter in effect relating to creditors’ rights generally, (b) general principles of equity (regardless of whether enforceability is considered in a proceeding at law or in equity), and (c) public policy considerations which may limit the rights of parties to obtain remedies.

6. With respect to any Stock Purchase Contracts (the “Offered Stock Purchase Contracts”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Stock Purchase Contracts has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Stock Purchase Contracts are to be sold pursuant to a firm commitment underwritten offering, an underwriting agreement with respect to the Offered Stock Purchase Contracts has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the Offered Stock Purchase Contracts and related matters; (v) the terms of the Offered Stock Purchase Contracts and of their issuance and sale have been duly established in conformity with the applicable Stock Purchase Contract Agreement so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company and the applicable Stock Purchase Contract Agent; (vi) the Preferred Stock or Common Stock relating to such Offered Stock Purchase Contracts have been duly issued and paid for in the manner contemplated in the Registration Statement and any prospectus supplement relating thereto; and (vii) the Offered Stock Purchase Contracts have been duly executed, delivered, countersigned, issued and sold in accordance with the provisions of the applicable Stock Purchase Contract Agreement in the manner contemplated in the Registration Statement or any prospectus supplement or term sheet relating thereto, the Offered Stock Purchase Contracts, when issued and sold in accordance with the

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applicable Stock Purchase Contract Agreement and the applicable purchase agreement or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, except to the extent that enforcement thereof may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or other similar laws now or hereafter in effect relating to creditors’ rights generally, (b) general principles of equity (regardless of whether enforceability is considered in a proceeding at law or in equity) and (c) public policy considerations which may limit the rights of parties to obtain remedies.

7. With respect to any Stock Purchase Units (the “Offered Stock Purchase Units”), when (i) the Registration Statement, as finally amended (including all necessary post-effective amendments), has become effective under the Act; (ii) an appropriate prospectus supplement or term sheet with respect to the Offered Stock Purchase Units has been prepared, delivered and filed in compliance with the Act and the applicable rules and regulations thereunder; (iii) if the Offered Stock Purchase Units are to be sold pursuant to a firm commitment underwritten offering, an underwriting agreement with respect to the Offered Stock Purchase Units has been duly authorized, executed and delivered by the Company and the other parties thereto; (iv) the Board of Directors, including any appropriate committee appointed thereby, and appropriate officers of the Company have taken all necessary corporate action to approve the issuance and terms of the Offered Stock Purchase Units and related matters; (v) the terms of the Offered Stock Purchase Units and the related Offered Stock Purchase Contracts and of their issuance and sale have been duly established in conformity with the applicable Stock Purchase Contract Agreement of which the Offered Stock Purchase Units are a component so as not to violate any applicable law, the Certificate of Incorporation or the By-Laws or result in a default under or breach of any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company and the applicable Stock Purchase Contract Agent; (vi) the Preferred Stock or Common Stock relating to such Offered Stock Purchase Contracts have been duly issued and paid for in the manner contemplated in the Registration Statement and any prospectus supplement relating thereto; and (vii) the Offered Stock Purchase Units have been duly executed, delivered, countersigned, issued and sold in accordance with the provisions of the applicable Stock Purchase Contract Agreement in the manner contemplated in the Registration Statement or any prospectus supplement or term sheet relating thereto, the Offered Stock Purchase Units, when issued and sold in accordance with the applicable Stock Purchase Contract Agreement and the applicable purchase agreement or any other duly authorized, executed and delivered valid and binding purchase or agency agreement, will be valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, except to the

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extent that enforcement thereof may be limited by (a) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or other similar laws now or hereafter in effect relating to creditors’ rights generally, (b) general principles of equity (regardless of whether enforceability is considered in a proceeding at law or in equity) and (c) public policy considerations which may limit the rights of parties to obtain remedies.

We hereby consent to the filing of this opinion with the Commission as an exhibit to the Registration Statement. We also hereby consent to the use of our name under the heading “Legal Matters” in the prospectus which forms a part of the Registration Statement. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission promulgated thereunder. 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 laws.

Very truly yours,

/s/ Skadden, Arps, Slate, Meagher & Flom LLP