Warfarin usually takes about four to five days to reach its full effect. Sometimes your doctor will start you on warfarin and wait for your INR to reach the desired level.

There are other times when it is important that your blood's ability to clot is reduced straight away. You will then be started on warfarin as well as injections of another anticoagulant that works immediately. These injections will usually only be for about four to five days until your INR is stable at the target level.