The main problem is that the controversy involves a lot of anecdote. Critics can't prove they've actually not been consuming some dangerous adulterants because

The main problem is that the controversy involves a lot of anecdote. Critics can't prove they've actually not been consuming some dangerous adulterants because they aren't testing their supply (since testing is pretty expensive). Here's at least one paper discussing kratom: https://www.ncbi.nlm.nih.gov/pubmed/22133323 https://www.ncbi.nlm.nih.gov/pubmed/22133323