Does Cosigning a student loan lower your credit score?

My sister needs me to cosign her loan. I have just graduted from college and worked myself up to a 700 FICO score (I know it's not perfect). I wonder if cosigning a $10K loan for four years in a row, will destroy what I have worked so hard for. I know I will do it either way, because I understand how important it is. But I guess I would like to know and not be surprised. Thank you in advance!