C

Stannous fluoride (SnF₂) quickly hydrolyzes and reacts with the phosphate present in saliva, precipitating as tin-hydroxyl-phosphate (Sn(OH)PO₄); this insoluble compound is what imparts the characteristic metallic taste noted after SnF₂ use [Rølla & Ellingsen, 1994, PMID 8021042].