I found out the hard way. Methylene blue is sometimes deliberately contaminated with arsenic, possibly to prevent us from benefiting from a safe effective product. The type I bought had a warning label not to be used in food producing fish. I called the store I bought it from, they said it did not have arsenic. This was a lie.
I suffered greatly from severe arsenic poisoning. I wish there was a way to be sure the source was pure, because aside from the arsenic, methylene blue was great stuff.
