Exhibit 5.1

[Letterhead of Skadden, Arps, Slate, Meagher & Flom LLP]

July 5, 2016

Express Scripts Holding Company

One Express Way

St. Louis, Missouri 63121

Ladies and Gentlemen:

We have acted as special counsel to Express Scripts Holding Company, a Delaware corporation (the “Company”), in connection with the public offering of (a) $1,000,000,000 aggregate principal amount of the Company’s 3.000% Senior Notes due 2023 (the “2023 Notes”), to be issued under the Indenture, dated as of November 21, 2011 (the “Base Indenture”), among the Company, certain subsidiaries of the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”), as supplemented by the Eighteenth Supplemental Indenture, dated as of July 5, 2016 (the “Eighteenth Supplemental Indenture”), among the Company, the Guarantors listed on Schedule I hereto (collectively, the “Guarantors”) and the Trustee, (b) $1,500,000,000 aggregate principal amount of the Company’s 3.400% Senior Notes due 2027 (the “2027 Notes”), to be issued under the Base Indenture, as supplemented by the Nineteenth Supplemental Indenture, dated as of July 5, 2016 (the “Nineteenth Supplemental Indenture”), among the Company, the Guarantors and the Trustee and (c) $1,500,000,000 aggregate principal amount of the Company’s 4.800% Senior Notes due 2046 (together with the 2023 Notes and the 2027 Notes, the “Notes”), to be issued under the Base Indenture, as supplemented by the Twentieth Supplemental Indenture, dated as of July 5, 2016 (the “Twentieth Supplemental Indenture” and, together with the Base Indenture, the Eighteenth Supplemental Indenture and the Nineteenth Supplemental Indenture, the “Indenture”), among the Company, the Guarantors and the Trustee. The Indenture provides that the Notes are to be guaranteed by the Guarantors (such guarantees, together with the Notes, 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”).

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July 5, 2016

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In rendering the opinions stated herein, we have examined and relied upon the following:

(a) the registration statement on Form S-3 (File No. 333-196442) of the Company and the Guarantors relating to the Securities and other securities of the Company filed on June 2, 2014 with the Securities and Exchange Commission (the “Commission”) under the Securities Act, allowing for delayed offerings pursuant to Rule 415 of the General Rules and Regulations under the Securities Act (the “Rules and Regulations”), and Post-Effective Amendment No. 1 thereto, including the information deemed to be a part of the registration statement pursuant to Rule 430B of the Rules and Regulations (such registration statement, as so amended, being hereinafter referred to as the “Registration Statement”);

(b) the prospectus, dated June 2, 2014 (the “Base Prospectus”), which forms a part of and is included in the Registration Statement;

(c) the preliminary prospectus supplement, dated June 29, 2016, relating to the offering of the Securities, in the form filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations (together with the Base Prospectus, the “Preliminary Prospectus”);

(d) the prospectus supplement, dated June 29, 2016, relating to the offering of the Securities, in the form filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations (together with the Base Prospectus, the “Prospectus”);

(e) an executed copy of the Underwriting Agreement, dated June 29, 2016 (the “Underwriting Agreement”), among the Company, the Guarantors and Citigroup Global Markets Inc., Credit Suisse Securities (USA) LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as representatives of the several Underwriters named therein (the “Underwriters”), relating to the sale by the Company and the Guarantors to the Underwriters of the Securities;

(f) an executed copy of the Base Indenture;

(g) an executed copy of the Eighteenth Supplemental Indenture;

(h) an executed copy of the Nineteenth Supplemental Indenture;

(i) an executed copy of the Twentieth Supplemental Indenture;

(j) the global certificates evidencing the Notes registered in the name of Cede & Co. (collectively, the “Note Certificates”) in the form delivered by the Company to the Trustee for authentication and delivery;

(k) each notation of guarantee (the “Guarantees”) of each of the Guarantors, endorsed on the Note Certificates;

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(l) an executed copy of the certificate of Timothy C. Wentworth, President and Chief Executive Officer of the Company, and Eric Slusser, Executive Vice President and Chief Financial Officer of the Company, dated the date hereof (the “Officer’s Certificate”);

(m) an executed copy of a certificate for each Opinion Party (as defined below) of the Secretary or Assistant Secretary, as applicable, of each Opinion Party, dated the date hereof (collectively, the “Secretary’s Certificates”);

(n) a copy of each Opinion Party’s articles of incorporation, certificate of incorporation, certificate of formation or articles of organization, as applicable, certified by the Secretary of State of each such Opinion Party’s applicable jurisdiction of formation or organization, as applicable, and certified pursuant to the Secretary’s Certificates, as applicable;

(o) a copy of the Company’s by-laws, as amended and in effect on the date hereof and certified pursuant to the Company’s Secretary’s Certificate;

(p) a copy of the by-laws, limited liability company agreement or partnership agreement, as applicable, in each case as amended and in effect as of the date hereof, of each Opinion Party Guarantor (as defined below), certified pursuant to the Secretary’s Certificates, as applicable;

(q) a copy of certain resolutions of the board of directors of the Company (the “Board of Directors”), adopted on June 21, 2016, and of the financing committee of the Board of Directors adopted on June 29, 2016, certified pursuant to the Company’s Secretary’s Certificate;

(r) copies of certain resolutions of the board of directors, partners or other managing body, as applicable, of each Opinion Party Guarantor, certified pursuant to the Secretary’s Certificates, as applicable;

(s) copies of certificates, each dated as of June 24, 2016, and a bringdown verification thereof, dated the date hereof, from the Secretary of State of the State of Delaware with respect to each Delaware Opinion Party’s (as defined below) existence and good standing in the State of Delaware (collectively, the “Delaware Certificates”);

(t) copies of a certificate, dated as of June 24, 2016, and a bringdown verification thereof, dated the date hereof, from the Secretary of the Commonwealth of the Commonwealth of Massachusetts with respect to the Massachusetts Opinion Party’s (as defined below) legal existence and good standing with the office of the Secretary of the Commonwealth of the Commonwealth of Massachusetts (the “Massachusetts Certificate”); and

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(u) a copy of a certificate, dated as of June 24, 2016, and a bringdown verification thereof, dated the date hereof, from the office of the Secretary of State of the State of New York with respect to each New York Opinion Party’s (as defined below) existence in the State of New York (collectively, the “New York Certificates”).

We have also examined originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and the Guarantors and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company, the Guarantors and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinions stated 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, the Guarantors and others and of public officials, including the facts and conclusions set forth in the Officer’s Certificate and the factual representations and warranties contained in the Transaction Agreements (as defined below).

We do not express any opinion with respect to the laws of any jurisdiction other than (i) the federal laws of the United States of America, (ii) the laws of the State of New York, including the Business Corporation Law of the State of New York (“NYBCL”), (iii) the General Corporation Law of the State of Delaware (the “DGCL”), the Delaware Limited Liability Company Act (the “DLLCA”), the Delaware Revised Uniform Partnership Act (“DRUPA”) and the Massachusetts Business Corporation Act (the “MBCA”), and (iv) 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 as to the effect of any non-Opined on Law on the opinions stated herein.

As used herein, (i) “Opinion Parties” means the Company and the Opinion Party Guarantors, (ii) “Opinion Party Guarantors” means the Delaware Opinion Party Guarantors, the Massachusetts Opinion Party Guarantor and the New York Opinion Party Guarantors, (iii) “Delaware Opinion Party Guarantors” means each of the entities listed on Schedule II hereto under the headings “Delaware Corporate Opinion Party Guarantors”, “Delaware Partnership Opinion Party Guarantor” and “Delaware LLC Opinion Party Guarantors”, (iv) “Massachusetts Opinion Party Guarantor” means the entity listed on Schedule II hereto under the heading “Massachusetts Opinion Party Guarantor,” and (v) “New York Opinion Party Guarantors” means the entities listed on Schedule II hereto under the heading “New York Opinion Party Guarantors.” “Non-Opinion Party Guarantors” means each of the Guarantors listed on Schedule III hereto. “Transaction Agreements” means the Indenture, the Note Certificates and the Guarantees.

Based upon the foregoing and subject to the qualifications and assumptions stated herein, we are of the opinion that:

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1. The Note Certificates 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 Note Certificates will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms under the laws of the State of New York.

2. The Guarantees of each Opinion Party Guarantor have been duly authorized by all requisite corporate action on the part of such Opinion Party Guarantor under the DGCL, NYBCL, DLLCA, DRUPA or MBCA, as applicable, and, when the Note Certificates are issued and delivered by the Company against payment therefor in accordance with the terms of the Underwriting Agreement and the Indenture, each Guarantee will constitute the valid and binding obligation of the applicable Guarantor, enforceable against such Guarantor in accordance with its terms under the laws of the State of New York.

The opinions stated herein are subject to the following qualifications:

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In addition, in rendering the foregoing opinions we have assumed that:

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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 also hereby consent to the reference to our firm under the heading “Legal Matters” in the Preliminary Prospectus and the Prospectus. 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 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 laws.

Very truly yours,

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

S.J.K.

Schedule I

Guarantors

I-2

Schedule II

Opinion Party Guarantors

II-2

Schedule III

Non-Opinion Party Guarantors