An in-vitro study found that fucoidan helps
metastasizing tumor cells to invade tissues.
Surely, that can't be a good thing!  How do
we know fucoidan would be safe if you have
an undiscovered cancer?

Exp Cell Res 1998 Mar 15;239(2):301-10
Sulfated glycosaminoglycans enhance tumor cell invasion
in vitro by stimulating plasminogen activation.

Metastasizing tumor cells invade host tissues by degrading
extracellular matrix constituents. We report here that the
highly sulfated glycosaminoglycans, heparin and heparan
sulfate, as well as the sulfated polysaccharide, fucoidan,
significantly enhanced tumor cell invasion in vitro into
fibrin, the basement membrane extract, Matrigel, or through
a basement membrane-like extracellular matrix. The
enhancement of tumor cell invasion was due to a stimulation
of the proteolytic cascade of plasminogen activation since
the effect required plasminogen activation and was
abolished by inhibitors of urokinase-type plasminogen
activator (uPA) or plasmin. Sulfated polysaccharides
enhanced five reactions of tumor-cell initiated plasminogen
activation in a dose-dependent manner. They amplified
plasminogen activation in culture supernatants up to 70-fold
by stimulating (i) pro-uPA activation by plasmin and
(ii) plasminogen activation by uPA. (iii) In addition,
sulfated polysaccharides partially protected plasmin
from inactivation by alpha 2-antiplasmin. Sulfated
polysaccharides also stimulated tumor-cell associated
plasminogen activation, e.g., (iv) cell surface pro-uPA
activation by plasmin and (v) plasminogen activation by
cell surface uPA. These results suggest that sulfated
glycosaminoglycans liberated by tumor-cell mediated
extracellular matrix degradation in vivo might amplify
pericellular plasminogen activation and locally enhance
tumor cell invasion in a positive feedback manner.