Non-Responders who do not meet the laboratory criteria for additional dosing will be discontinued from the study and treated for RLS as deemed appropriate by the referring physician. Subjects who are Non-Responders at the end of Treatment Phase I and met the necessary laboratory criteria (ferritin <300 ng/mL and a TSAT <45%) will consented for enrollment in Treatment Phase II of the study. These subjects will receive the first of two unblinded doses of FCM (undiluted slow IV push 750 mg 100 mg/minute) on Day 0 of Phase II, which will occur within 7 days of the completing Treatment Phase I visit. The subjects will then receive the second dose of FCM (undiluted slow IV push 750 mg 100 mg/minute) on Day 7. All treated subjects will have blood samples taken for hematology, chemistries and iron indices on Day 14. Treatment efficacy (IRLSS score), medication review and adverse events assessment by phone will be done on Day 28. All subjects will return to the clinic on Day 42 for end of study assessments. Subjects completing Treatment Phase II will be re-evaluated on Day 42 and defined as either a Responder or Non-Responder, using the same criteria described above. Subjects deemed as treatment Non-Responders will be discontinued from the study after final assessments are complete.