California Code, Business and Professions Code - BPC § 2274

(a) The use by any licensee of any certificate, of any letter, letters, word, words, term, or terms either as a prefix, affix, or suffix indicating that he or she is entitled to engage in a medical practice for which he or she is not licensed constitutes unprofessional conduct.

(b) Nothing in this section shall be construed to prohibit a physician and surgeon from using the designations specified in this section if he or she has been issued a retired license under Section 2439.