Composition for use in anticoagulant treatment and determined on the basis of an optimal dosage D0 of an antivitamin K for obtaining and maintaining the value of a target INR Io previously defined by the prescriber according to the treated patient's disease, characterised in that the optimal dosage D0 of said antivitamin K is obtained using a method consisting of: - (a) recording the value I o of the target optimal INR, - (b) recording the INR values measured in a given period T 1 using an overdosage D max of said antivitamin K previously defined by the prescriber above the recommended dosage, in order to determine a first slope P 1 of the INR values as a function of the time T 1, - (c) on the basis of the slope P1, extrapolating and recording an INR value I max theoretically obtained from the day when the value I o of the target INR is obtained and after a predetermined period To, - (d) recording the INR values measured in a given period T 2 using an underdosage D min of said antivitamin K previously defined by the prescriber below the recommended dosage, in order to determine a second slope P 2 of the INR values as a function of the time T 2, - (e) on the basis of the slope P2, extrapolating and recording an INR value I min theoretically obtained from the day when the value I o of the target INR is obtained and after a predetermined period To, - (f) calculating on the basis of the difference between the INR values I max and I min theoretically calculated, a dosage correction factor of said antivitamin K expressed according to the following formula: F = I max - I min D max - D min - (g) in order to determine on the basis of the difference between the values I 0 of the optimal target INR and I min of the calculated minimum INR, and the dosage correction factor F of said antivitamin K, the adequate dosage D0 for obtaining and maintaining the value Io of the target INR previously defined according to the following relation: D 0 = D min + I 0 - I min F - (a) recording the value I o of the target optimal INR, - (b) recording the INR values measured in a given period T 1 using an overdosage D max of said antivitamin K previously defined by the prescriber above the recommended dosage, in order to determine a first slope P 1 of the INR values as a function of the time T 1, - (c) on the basis of the slope P1, extrapolating and recording an INR value I max theoretically obtained from the day when the value I o of the target INR is obtained and after a predetermined period To, - (d) recording the INR values measured in a given period T 2 using an underdosage D min of said antivitamin K previously defined by the prescriber below the recommended dosage, in order to determine a second slope P 2 of the INR values as a function of the time T 2, - (e) on the basis of the slope P2, extrapolating and recording an INR value I min theoretically obtained from the day when the value I o of the target INR is obtained and after a predetermined period To, - (f) calculating on the basis of the difference between the INR values I max and I min theoretically calculated, a dosage correction factor of said antivitamin K expressed according to the following formula: F = I max - I min D max - D min - (g) in order to determine on the basis of the difference between the values I 0 of the optimal target INR and I min of the calculated minimum INR, and the dosage correction factor F of said antivitamin K, the adequate dosage D0 for obtaining and maintaining the value Io of the target INR previously defined according to the following relation: D 0 = D min + I 0 - I min F