People take warfarin in order to thin the blood and prevent clotting.  As probiotics help the production of Vitamin K, which can thicken the blood, there is concern that taking probiotics will reverse the effects of taking warfarin. 

However, there is currently no published evidence showing that probiotics thicken the blood.  Although probiotics have indeed been shown to help production of Vitamin K, the body does not absorb more levels of Vitamin K than required.