A known side effect of bismuth salts is harmless and reversible darkening of tongue and stool by formation of bismuth sulfite.[3] Other side effects of bismuth containing combination therapies are often difficult to assign to a specific component.[citation needed]
Bismuth subcitrate potassium is a salt of bismuth (Bi3+), potassium (K+) and citrate (C6H4O74−) in a molar ratio of about 1:5:2, with 3 moles of water. It contains about 25.6% (mass percent) bismuth, which is the active moiety, and 22.9% potassium.[3][4] Other sources give somewhat different ratios of the constituents.