How bad is it to eat licorice while taking coumadin (warfarin)? I have trouble keeping my dad away from sweets

Can decrease effect. Licorice can decrease the effectiveness of warfarin or coumadin. That means, the blood will not be thin enough and can lead to higher chance of clot forming. You can refer to the following link for more information. http://www.ptinr.com/warfarin-you/dietary-food-beverage/food-interactions/licorice-and-warfarin.