Methylene Blue is an FDA approved drug for use in humans. It is available by prescription and it is a relatively cheap drug.
The fish/aquarium grade Methylene Blue is available over the counter to anyone and it is dirt cheap.
Methylene Blue appears to have some fairly good antimicrobial properties. It has been used in humans as a treatment for malaria and urinary tract infections.
Methylene Blue is also used as an anti-fungal in fish tanks/aquariums.