Exhibit 5.1

Opinion of Ropes & Gray, LLP

regarding securities being registered

November 8, 2013

La Jolla Pharmaceutical Company

4660 La Jolla Village Drive, Suite 1070

San Diego, California 92122

Ladies and Gentlemen:

We have acted as counsel to La Jolla Pharmaceutical Company, a California corporation (the “Company”) in connection with the Registration Statement on Form S-1 (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”) for the registration of 142,857,139 shares of the common stock, par value $0.0001 per share, of the Company (the “Securities”).

In connection with this opinion letter, we have examined such certificates, documents and records and have made such investigation of fact and such examination of law as we have deemed appropriate in order to enable us to render the opinions set forth herein. In conducting such investigation, we have relied, without independent verification, upon certificates of officers of the Company, public officials and other appropriate persons.

The opinions expressed below are limited to the Delaware General Corporation Law.

Based upon and subject to the foregoing, we are of the opinion that the Securities have been duly authorized and are validly issued, fully paid and non-assessable.

We hereby consent to your filing this opinion as an exhibit to the Registration Statement and to the use of our name therein and in the related prospectus under the caption “Legal Matters.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Ropes & Gray LLP

Ropes & Gray LLP