That's a common and very acceptable concern and there's not great answers for it. Mostly a place will do multi-vail testing for purity, sterility, endotoxin and heavy metals. The idea is that the sample is sufficient to show safety. Many are rightfully dubious of this but still thousand and thousands of people taking it.

I'm using reta myself, but honestly it's because sema/tirz isn't allowed for me either. If you have medical tirz available, you should have some serious reasons to jump into the unregulated shark tank.