Instability of Sodium Monofluorophosphate in Effervescent Tablets

Sodium monofluorophosphate (MFP, CAS 10163-15-2), widely used as an anti-caries agent and in the prevention or treatment of osteoporosis, is sensitive to moisture and acid. This research presents results of a study of the stability of MFP in tablets from four different pharmaceutical laboratories. With an initial water content of 2.9 to 4.0%, the pH of water solutions of three brands of non-effervescent tablets after disaggregation in water ranged from 5.2 to 7.4. The shelf half-life of these tablets ranged from 24 to 233 months. By contrast, effervescent tablets provided conditions that increased the instability of MFP: they produced the lowest pH (4.4) when dissolved in water and had the greatest water content (7% of tablet initial weight). They also had the shortest half-life (4.5 months). Kept in their original containers, effervescent tablets lost their total fluoride content at the rate of 41.0±8.9 μg F–/tablet/day (p<0.01), with half of this loss occurring as hydrofluoric acid at a rate 21±0.3 μg F–/tablet/day (p<0.01).