What precautions should be taken when taking warfarin?

Adult usual dosage: 10 mg once daily for three consecutive days. During the first one to two days, prothrombin activity primarily reflects the decline of the short-lived coagulation factor VII, and anticoagulant effect during this period is unstable. Approximately three days later, factors II, IX, and X become substantially depleted, and the full anticoagulant effect becomes apparent. Prothrombin time (PT) more accurately reflects the degree of reduction in vitamin K–dependent clotting factors and thus serves as a reliable guide for determining the maintenance dose.

Use in Pregnant and Lactating Women:

1. Warfarin readily crosses the placenta and is teratogenic. Use during pregnancy may cause “fetal warfarin syndrome,” with an incidence ranging from 5% to 30%. Administration during the late stages of pregnancy increases the risk of fetal hemorrhage and intrauterine fetal demise. Therefore, warfarin is contraindicated during the first trimester (first three months) and third trimester (last three months) of pregnancy. In pregnant women with hereditary thrombophilia requiring warfarin therapy, low-dose heparin may be administered alongside close laboratory monitoring.

2. Small amounts of warfarin are excreted into breast milk. In lactating women receiving daily doses of 5–10 mg, maternal plasma concentrations typically range from 0.48 to 1.8 μg/mL; concentrations in breast milk and infant plasma are extremely low, resulting in minimal clinical impact on the infant.