The GI who diagnosed my Blasto infection prescribed me Bactrim (which is the drug you are talking about) after I refused to take another round of Flagyl. Jackie Delaney's badbugs site says that there are a lot of failures associated with Bactrim. I never took it but instead opted for the nitazoxanide/furazolidone/secnidazole cocktail. If it's any help, the GI who gave me the Bactrim said it was successful in over 90% of the Blasto cases it was used. However, when I asked him where he got this information, he didn't respond.
Good luck. You should also know Bactrim is a sulfa type drug and many people have severe allergic reactions to it. I really hope it works for you though.