California Code, Labor Code - LAB § 3209.10

(a) Medical treatment of a work-related injury required to cure or relieve the effects of the injury may be provided by a state licensed physician assistant or nurse practitioner, acting under the review or supervision of a physician and surgeon pursuant to standardized procedures or protocols within their lawfully authorized scope of practice. The reviewing or supervising physician and surgeon of the physician assistant or nurse practitioner shall be deemed to be the treating physician. For the purposes of this section, “medical treatment” includes the authority of the nurse practitioner or physician assistant to authorize the patient to receive time off from work for a period not to exceed three calendar days if that authority is included in a standardized procedure or protocol approved by the supervising physician. The nurse practitioner or physician assistant may cosign the Doctor's First Report of Occupational Injury or Illness. The treating physician shall make any determination of temporary disability and shall sign the report.

(b) The provision of subdivision (a) that requires the cosignature of the treating physician applies to this section only and it is not the intent of the Legislature that the requirement apply to any other section of law or to any other statute or regulation. Nothing in this section implies that a nurse practitioner or physician assistant is a physician as defined in Section 3209.3.