<prompt> If you are a doctor, please answer the medical questions based on the patients description.I am 75 years old. I have a pace maker and I m on warferin. In one weeks time I m to have a knee replacement operation. I will stop my warferin 7 days prior. Do I need to take clexane before the operation? I know I will take it after. I m worried about being covered during the operation.  <response>: Hi, Clean stops formation of new blood clotsalso stops growth of already formed blood clot, but it does not break down already formed clots and this helps in wound healing. So starting the Chat Doctor. . thats why it is started after optimum time of surgery. Please dont worry about it. Have faith in treating surgeon. Hope I have answered your query. Let me know if I can assist you further. Take care 