Many types of brown seaweed contain the carbohydrate fucoidan, according to Memorial Sloan Kettering Cancer Center. Research indicates that fucoidan stops cancer cells from growing, slows blood clotting, helps the immune system, protects neurons and fights viruses. However, as of 2015, fucoidan has not gone though any testing on humans.
Although some sources suggest that fucoidan is helpful for treating allergies, lowering cholesterol levels and decreasing blood pressure, no evidence exists to support these claims, reports MSKCC. In addition, it is possible that fucoidan could increase the effects of blood-thinning drugs such as warfarin and heparin. In this case, taking fucoidan could result in blood not clotting properly.