Caution

Lettuce is a very safe vegetable and has no adverse reaction reported so far. There is only one condition in which large quantities of lettuce are contraindicated: For patients undergoing anticoagulant therapy with vitamin K antagonists (such as warfarin). The high amount of vitamin K, may interfere with the therapy, possibly leading to blood clots formation (thrombosis) and related complications. So be aware if youre on such medication.