To exercise, in this case, is just a formal way of saying to use. And the word caution is straightforward enough. So all together, it means that patients whose livers cannot function normally should be careful (of the amount of the vitamin intake).

Your warning on the vitamin bottle is sayinig if someone has a liver problem, they should be careful about taking those vitamins....it could make their liver problem worse. This means "be cautious", "ask your doctor if it's okay", find out what you're doing.....these vitamins may not be good for you. Take caution, take care, take a warning!

忍让 = to exercise patience "Exercise" can be used with some words to have this meaning: exercise caution exercise restraint exercise power exercise authority Look up these words in a dictionary to get a grasp of what "exercise" means here. Here it means: be cautious