Colistin is a dry powder that was originally formulated to be used as an IV antibiotic. Off label is is used as an inhaled antibiotic. It's like TOBI that way, but a little more cumbersome to use since you are required to mix the dry powder formula with sterile water to make the formulation that is then nebulized. It is nebulized twice daily (again, similar to TOBI). We have alternated TOBI and colistin several years with some benefit. We have alternated with 2 week cycles and 1 month cycles. We have not had any problems with irritation, but our Dr. has told us that if it is irritating we can add more sterile water to the treatment. Hope this helps and good luck!