Researcher after researcher has claimed that serrapeptase has no side effects, and that is overwhelmingly true.
Anecdotally, side effects for the serrapeptase enzyme have been all but non-existent, and in most clinical trials that have been reviewed, no side effects have been reported.
There were, however, a very few, very minor side effects reported in one clinical trial, a case of minor diarrhea for one participant and a headache for another. (See Where to Buy Serrapeptase for more information.)
And since it does have the effect of dissolving fibrin, which makes the blood less “sticky” and thus helps prevent clotting by “thinning” it, anyone taking medication for blood thinning should definitely discuss the situation with their doctors.