Iron deficiency has a high prevalence in colorectal cancer patients ranging at ca. 60%. About 70% of these patients suffer from iron deficiency anemia (IDA) which adds both physical and cognitive impediments to an already straining chemotherapy. Moreover, a chronic disease like cancer often results in a reduced availability of iron for the body. In clinical practice iron substitution is usually administered orally. Due to low resorption rates, frequent gastric side effects and thus poor patient compliance a parenteral substitution seems to be a better option in terms of efficacy. In the framework of a randomized multicenter clinical trial ('FerInject') a comparison of efficacy parameters of parenteral vs. oral iron substitution will now be conducted in order to identify the best treatment form for clinical practice in oncology. Furthermore detailed quality of life-data (QoL) will be collected in both treatment arms for effect comparison.

Inclusion Criteria:

1. Metastatic or inoperable colorectal carcinoma. No curative therapy available.
2. Current palliative chemotherapy. Patients under conversion therapy must not be enrolled to this study.
3. Iron deficiency anemia: hemoglobin ≤ 10.5 g/dl and transferrin saturation < 20 % and/or serum ferritin < 20 ng/ml
4. Male and female patients aged ≥ 18 years; maturity
5. ECOG ≤ 2
6. Written informed consent
7. Life expectancy > 6 months
8. Body weight ≥ 40 kg

Exclusion Criteria:

1. Oral or intravenous iron substitution within the last 4 weeks
2. Age < 18 years or body weight < 40 kg
3. Absorption dysfunction due to short bowel syndrome or after gastric resection
4. Therapy with recombinant erythropoietin within the last 4 weeks
5. Chronic diarrhea
6. Chronic inflammatory bowel disease
7. Ferritin > 800 mg/dl at baseline
8. Hypersensitivity or contraindication to ferric carboxymaltose or iron (II) glycine sulphate complex
9. Known vitamin B12 or folic acid anemia
10. Necessary total parenteral nutrition
11. Participation in another interventional study
12. Pregnancy or lactation

No IPD will be shared.