Determining optimal dosage (D o) of vitamin K, comprises: recording value I oof optimal target INR recording international normalized ratio (INR) values measured in a time T 1 from a gradient P 1, extrapolating and recording a I maxvalue of INR theoretically reached from the day recording INR values measured in a period T 2 from the gradient P2, extrapolating and recording a INR I minivalue theoretically reached from the day calculating difference between INR I maxiand I minivalues and a factor F and determining the difference between values I oand I mini. Determining optimal dosage (D o) of vitamin K for anticoagulant therapy for achieving and constantly maintaining the value of international normalized ratio (INR) target I opreviously determined by the prescriber according to the pathology of the treated patient, comprises: (a) recording the I ovalue of the optimal target INR (b) recording the INR values measured in a given time (T 1) by an over dose (D maxi) of the antivitamin K previously determined by the prescriber over the recommended dosage, for determining a first gradient (P 1) of INR values according to the time T 1 (c) from the gradient P 1, extrapolating and recording a I maxvalue of INR theoretically reached from the day where the value I oof the target INR is reached and at the end of a determined period T o recording the INR values measured in a given period T 2from an underdosing D miniof the antivitamin K fixed by the prescriber below the recommended dosage, for determining a second gradient P 2of values INR according to the time T 2 (d) from the gradient P2, extrapolating and recording a INR I minivalue theoretically reached from the day where the value I oof the target INR is reached and at the end of the fixed period T 0 (e) calculating the difference between the theoretically calculated INR I maxiand I minivalues, a factor F for correcting dose of the antivitamin K expressed by a formula as given in the specification (g) determining the differe
