This product has little in the way of evidence for effectiveness.
(that is if you subscribe to medical studies). Therefore long term safety data is not available for this product.

It is a peptide and thus subject to acid destruction in the stomach.
Using a product therefore that is enteric coated may be best.
(or some other way to protect the enzyme from stomach acid destruction--as Doctor's Best claims).

http://en.wikipedia.org/wiki/Serratiopeptidase

The warnings I could find are warnings of blood thinning...so do not use with aspirin or other NSAIDs or RX drugs for blood thinning(drugs that reduce blood clotting).

The list of inflammatory conditions that supposedly helped are quite similar to those that Olive Leaf Extract is used for.
You might want to read this thread about Olive Leaf and the links on it:
http://neurotalk.psychcentral.com/sh...ighlight=olive
http://www.lef.org/magazine/mag2013/...Extract_01.htm

I don't have any personal experience with serrapeptase, but I am working with Olive Leaf.... and having some progress with it.
My son just loves this supplement and uses the 20% form, that he gets at Vitamin Shoppe.
http://www.vitaminshoppe.com/p/plnt-...5#.VBgf8hYs1BU

I am still using the 6%...and find my own Herx reactions to it when used every day rather intense and strange. My son does not Herx on his brand. He has lots of upper respiratory problems, and my issues are primarily GI ones. But we discussed his experience with it on the long drive home Sat and he absolutely loves the olive leaf!