Probenecid (a drug treating gout) impairs riboflavin absorption, and propantheline bromide (a drug treating peptic ulcers) reportedly both delays and increases absorption. Phenothiazines (antipsychotic drugs) increase the excretion of riboflavin, thus lowering serum levels; and oral contraceptives may also decrease serum levels. Supplementation should be discussed with a health care provider if these medications are being used. Absorption of riboflavin is improved when taken together with other B vitamins and vitamin C.
Bratman, Steven, and David Kroll. Natural Health Bible. Prima Publishing, 1999.