The present invention provides a side effect inhibitor of drug therapy for colorectal cancer using 5-fluorouracil, comprising fucoidan or a fucoidan-containing material a method for inhibiting side effects of drug therapy for colorectal cancer using 5-fluorouracil, which comprises administering fucoidan or a fucoidan-containing material to a colorectal cancer patient and use of fucoidan or a fucoidan-containing material for the production of a side effect inhibitor of drug therapy for colorectal cancer using 5-fluorouracil.