Wheatgrass juice is green food, not a drug. Certain blood thinners restrict green food intake and wheatgrass juice is not appropriate for these individuals.
The most common blood thinner, Coumadin (generic Warfarin), is one that restricts green food intake.
Please note that many people are replacing Coumadin (generic Warfarin) with newer blood thinners that have no dietary restrictions. Whether or not this is appropriate for you is a question for your doctor or pharmacist.