High Ingestion Rate of Iodine from Povidone-Iodine Mouthwash

Iodine-based mouthwash and throat sprays contain povidone iodine (PVP-I) for disinfection. PVP-I mouthwash has been commonly used for decades in Japan and other countries and frequent and/or prolonged use of PVP-I mouthwash can induce transient hypothyroidism. To assess the amount of iodine ingested from an oral rinse, 22 healthy adult volunteers (mean age: 48.1, 29–70 years) were recruited for the study. The subjects were instructed to rinse for 15 s three times with 20 mL of commercially available PVP-I mouthwash diluted into 0.23% or pure water. This method is a standardized method of gargling recommended by the manufacturers. The total iodine in the PVP-I mouthwash was measured with inductively coupled plasma-mass spectrometry. Although the 7% PVP-I mouthwash contains 7 mg of effective iodine/mL, 24.3 mg/mL of iodine was detected in the solution. The median value and ratio of the total iodine ingested were 5.0 mg (range: 2.6–10.8 mg) and 20.5% (range: 10.6–44.5%), respectively. The iodine species released from the PVP-I mouthwash are effective iodine (PVP・nHI(3), I(3)(−), and I(2)) and I(−); however, the amount and types of iodine actually absorbed into the bloodstream are unknown. PVP-I mouthwash should be used carefully since around 5 mg of iodine could theoretically enter the body with one gargle which exceeds the tolerable upper intake level of iodine for adults. This study was prospectively registered to University Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR) on March 29, 2021, with the study ID of UMIN000043770.