A method for determining from a blood sample of a patient suffering from cancer the dose D(n+1) of 5-fluorouracile (5-FU) for the next cycle of treatment (n+1), wherein • each treatment cycle i comprises: - 0-500 mg/m 2 of 5-fluorouracile (5-FU) administered in a bolus, - 0-600 mg/m 2 of folinic acid or a salt thereof, - a dose D(i) (in mg/m 2) of 5-FU administered in a continuous infusion of 43 to 49 hours, and - 0-500 mg/m 2 of irinotecan; and • said blood sample has been taken from said patient in previous treatment cycle n at least 1 hour after the beginning of the 5-FU perfusion and before the end of said perfusion, • said method comprising: • dosing in vitro the 5-FU plasmatic concentration ([5-FU]) in the blood sample • calculating D(n+1) depending on D(n) using the following decision scheme: - if [5-FU] < 100 μg/L, then D(n+1) = D(n) × 1.40, - if 100 ≤ [5-FU] < 200 μg/L, then D(n+1) = D(n) × 1.30, - if 200 ≤ [5-FU] < 300 μg/L, then D(n+1) = D(n) × 1.20, - if 300 ≤ [5-FU] < 400 μg/L, then D(n+1) = D(n) × 1.10, - if 400 ≤ [5-FU] < 500 μg/L, then D(n+1) = D(n) × 1.05, - if 500 ≤ [5-FU] < 600 μg/L, then D(n+1) = D(n) × 1.025, - if 600 ≤ [5-FU] ≤ 650 μg/L, then D(n+1) = D(n), - if 650< [5-FU] < 700 μg/L, then D(n+1) = D(n) × 0.975, - if 700 ≤ [5-FU] < 800 μg/L, then D(n+1) = D(n) × 0.95, - if 800 ≤ [5-FU] < 900 μg/L, then D(n+1) = D(n) × 0.90, - if ≥ 900 [5-FU], then D(n+1) = D(n) × 0.80. • each treatment cycle i comprises: - 0-500 mg/m 2 of 5-fluorouracile (5-FU) administered in a bolus, - 0-600 mg/m 2 of folinic acid or a salt thereof, - a dose D(i) (in mg/m 2) of 5-FU administered in a continuous infusion of 43 to 49 hours, and - 0-500 mg/m 2 of irinotecan; and - 0-500 mg/m 2 of 5-fluorouracile (5-FU) administered in a bolus, - 0-600 mg/m 2 of folinic acid or a salt thereof, - a dose D(i) (in mg/m 2) of 5-FU administered in a continuous infusion of 43 to 49 hours, and - 0-500 mg/m 2 of irinotecan; and • said blood sample has been taken from said patient in previous treatment cycle n at least 1 hour after the beginning of the 5-FU perfusion and before the end of said perfusion, • said method comprising: • dosing in vitro the 5-FU plasmatic concentration ([5-FU]) in the blood sample • calculating D(n+1) depending on D(n) using the following decision scheme: - if [5-FU] < 100 μg/L, then D(n+1) = D(n) × 1.40, - if 100 ≤ [5-FU] < 200 μg/L, then D(n+1) = D(n) × 1.30, - if 200 ≤ [5-FU] < 300 μg/L, then D(n+1) = D(n) × 1.20, - if 300 ≤ [5-FU] < 400 μg/L, then D(n+1) = D(n) × 1.10, - if 400 ≤ [5-FU] < 500 μg/L, then D(n+1) = D(n) × 1.05, - if 500 ≤ [5-FU] < 600 μg/L, then D(n+1) = D(n) × 1.025, - if 600 ≤ [5-FU] ≤ 650 μg/L, then D(n+1) = D(n), - if 650< [5-FU] < 700 μg/L, then D(n+1) = D(n) × 0.975, - if 700 ≤ [5-FU] < 800 μg/L, then D(n+1) = D(n) × 0.95, - if 800 ≤ [5-FU] < 900 μg/L, then D(n+1) = D(n) × 0.90, - if ≥ 900 [5-FU], then D(n+1) = D(n) × 0.80. • dosing in vitro the 5-FU plasmatic concentration ([5-FU]) in the blood sample • calculating D(n+1) depending on D(n) using the following decision scheme: - if [5-FU] < 100 μg/L, then D(n+1) = D(n) × 1.40, - if 100 ≤ [5-FU] < 200 μg/L, then D(n+1) = D(n) × 1.30, - if 200 ≤ [5-FU] < 300 μg/L, then D(n+1) = D(n) × 1.20, - if 300 ≤ [5-FU] < 400 μg/L, then D(n+1) = D(n) × 1.10, - if 400 ≤ [5-FU] < 500 μg/L, then D(n+1) = D(n) × 1.05, - if 500 ≤ [5-FU] < 600 μg/L, then D(n+1) = D(n) × 1.025, - if 600 ≤ [5-FU] ≤ 650 μg/L, then D(n+1) = D(n), - if 650< [5-FU] < 700 μg/L, then D(n+1) = D(n) × 0.975, - if 700 ≤ [5-FU] < 800 μg/L, then D(n+1) = D(n) × 0.95, - if 800 ≤ [5-FU] < 900 μg/L, then D(n+1) = D(n) × 0.90, - if ≥ 900 [5-FU], then D(n+1) = D(n) × 0.80. - if [5-FU] < 100 μg/L, then D(n+1) = D(n) × 1.40, - if 100 ≤ [5-FU] < 200 μg/L, then D(n+1) = D(n) × 1.30, - if 200 ≤ [5-FU] < 300 μg/L, then D(n+1) = D(n) × 1.20, - if 300 ≤ [5-FU] < 400 μg/L, then D(n+1) = D(n) × 1.10, - if 400 ≤ [5-FU] < 500 μg/L, then D(n+1) = D(n) × 1.05, - if 500 ≤ [5-FU] < 600 μg/L, then D(n+1) = D(n) × 1.025, - if 600 ≤ [5-FU] ≤ 650 μg/L, then D(n+1) = D(n), - if 650< [5-FU] < 700 μg/L, then D(n+1) = D(n) × 0.975, - if 700 ≤ [5-FU] < 800 μg/L, then D(n+1) = D(n) × 0.95, - if 800 ≤ [5-FU] < 900 μg/L, then D(n+1) = D(n) × 0.90, - if ≥ 900 [5-FU], then D(n+1) = D(n) × 0.80.