There are only 2 groups that cannot use wheatgrass.
Those with a grass allergy cannot use wheatgrass juice. It is rarely seen but would present as stomach cramping.
Coumadin (Warfarin) Blood Thinner Users
Like all greens, wheatgrass juice has Vitamin K.
Warfarin is a Vitamin K blocker and by introducing additional Vitamin K, it can stop you from achieving your desired INR (international normalized ratio).
If you are using Coumadin, your doctor and pharmacist should have instructed you to avoid all green foods including spinach, broccoli, leaf lettuce as well.
Keywords: contraindicated, contraindications