Critères d'inclusion : - Patient having either:
-> a metastatic head and neck cancer or in locoregional relapse, proved histologically, authorizing an oral feeding or by gastrostomy or nasogastric tube,
-> a metastatic colorectal cancer, histologically proved, wild-type RAS
- Patient who must be treated by anti-EGFR for this cancer
- Age ≥ 18 years
- Performance Status = 0, 1 or 2
- Patient affiliated to a social security scheme
- Patient who have given written consent prior to any specific study-related procedure
Critères de non-inclusion : - Cerebral metastasis
- Previous anti-EGFR treatment
- Patient requiring exclusive parenteral nutrition and hydration
- Concomitant treatment by radiotherapy
- Presence of another invasive cancer, other than head and neck or digestive, except basal cell carcinoma or intracervical neoplasia treated
- Presence of hypomagnesemia at randomization
- Ongoing oral or intravenous magnesium supplementation within 2 weeks before randomization
- Patient with grade III or IV diarrhea within 2 weeks before randomization
- Patient who have had a jejunostomy or ileostomy
- Patient with constitutional tubulopathy
- Patient with chronic renal insufficiency (MDRD Clarity <60 mL / min)
- Pregnancy or breast-feeding
- Persons deprived of their liberty or under guardianship
- Dementia, mental impairment or psychiatric pathology that may compromise the patient's informed consent and / or adherence to the protocol and follow-up of the trial
- Patient who can not follow protocol for psychological, social, family or geographical reasons