This Phase I human study in 16 healthy volunteers compared a 500 mg oral dose of methylene blue against a 50 mg intravenous dose in a randomized within-subject crossover, specifically to measure how much of an oral dose reaches circulation. Plasma exposure after oral dosing was high relative to intravenous, and the study reported high absolute oral bioavailability for the aqueous oral formulation. The work was done in the context of malaria treatment, where the authors wanted to rule outlow absorption as a cause of treatment failure. This is human pharmacokinetic evidence about absorption, not a study of clinical outcomes.
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Oral methylene blue shows high absolute bioavailability, suggesting absorption is unlikely to limit systemic exposure at standard doses.