Bismuth subsalicylate has the empirical chemical formula of C7H5BiO4, and it is a colloidal substance obtained by hydrolysis of bismuth salicylate (Bi(C6H4(OH)CO2)3). The actual structure[clarification needed] is unknown and the formulation is only approximate.[dubious ]
There are some adverse effects. It can cause a black tongue and black stools in some users of the drug, when it combines with trace amounts of sulfur in saliva and the colon to form bismuth sulfide. Bismuth sulfide is a highly insoluble black salt, and the discoloration seen is temporary and harmless.
Children should not take medication with bismuth subsalicylate while recovering from influenza or chicken pox, as epidemiologic evidence points to an association between the use of salicylate-containing medications during certain viral infections and the onset of Reye's syndrome. For the same reason, it is typically recommended that nursing mothers not use medication containing bismuth subsalicylate because small amounts of the medication are excreted in human breast milk, and these pose a theoretical risk of Reye's syndrome to nursing children.
Salicylates are very toxic to cats, and thus bismuth subsalicylate should not be administered to cats.
The British National Formulary does not recommend bismuth containing antacids (unless chelated), cautioning that absorbed bismuth can be neurotoxic, causing encephalopathy, and that such antacids tend to be constipating.