Sodium Ascorbyl Phosphate
Also known as Sodium Ascorbyl Phosphate, SAP, Sodium L-Ascorbyl-2-Phosphate, Sodium Ascorbyl Monophosphate, L-Ascorbic acid 2-(dihydrogen phosphate) trisodium salt, Stay-C 50
“CIR Expert Panel says: safe as used in cosmetics.”
Sodium Ascorbyl Phosphate (SAP; CAS 66170-10-3; C6H6Na3O9P) is a water-soluble, phosphorylated trisodium salt of L-ascorbic acid (vitamin C) used in cosmetics as a stable antioxidant, skin-brightening active, and anti-acne ingredient. SAP functions as a prodrug — converted to free ascorbic acid in skin by endogenous phosphatases. The CIR Expert Panel assessed SAP together with L-Ascorbic Acid, Calcium Ascorbate, Magnesium Ascorbate, Magnesium Ascorbyl Phosphate, and Sodium Ascorbate in a single 2005 group report and concluded the entire group safe as used in cosmetic formulations (Elmore, IJT 24(Suppl. 2):51-111, 2005; PMID 16154915), confirmed in the December 2025 QRT (Finding=S). Beyond the standard antioxidant/collagen-cofactor activity shared with other vitamin C derivatives, SAP has the most distinctive evidence base for treating acne vulgaris among ascorbate derivatives: a 1% SAP solution achieved a 5-log reduction of P. acnes in vitro after 8 hours, and 5% SAP lotion produced a 48.82% reduction in inflammatory acne lesions over 8 weeks in a randomized comparison vs. retinol (Klock et al., 2005, PMID 18492184; Ruamrak et al., 2009, PMID 19134126), with a separate 12-week double-blind RCT vs. vehicle confirming statistically significant improvement on all measured parameters (Woolery-Lloyd et al., 2010, PMID 20367669). SAP is stable in the pH 5.0-7.5 range that aligns with native skin pH — markedly more formulation-tolerant than free ascorbic acid (which requires pH below 3.5). A 2021 head-to-head emulgel trial showed comparable anti-wrinkle efficacy to ascorbic acid at the same 5% concentration with the formulation advantage of neutral pH (Mohammadi et al., 2021, PMID 32383548).
Superior pH compatibility vs. free ascorbic acid: SAP is stable across pH 5.0-7.5, the range matching native skin pH and most cosmetic emulsions, eliminating the low-pH stinging and rapid oxidation that constrain L-ascorbic acid formulations to pH below 3.5
Demonstrated anti-acne efficacy: 1% SAP solution achieved a 5-log reduction of P. acnes in vitro after 8 hours; 5% SAP lotion reduced inflammatory acne lesions by 48.82% at 8 weeks, comparable to 0.2% retinol (49.50%) without retinoid irritation (Klock et al. 2005, PMID 18492184; Ruamrak et al. 2009, PMID 19134126); a separate randomized double-blind controlled trial of 5% SAP lotion vs. vehicle for 12 weeks demonstrated statistically significant improvement on all measured parameters (Woolery-Lloyd et al. 2010, PMID 20367669)
Vitamin C prodrug delivery: enzymatically cleaved to L-ascorbic acid by cutaneous phosphatases, delivering tyrosinase inhibition (skin-brightening), collagen-synthesis cofactor activity, and ROS-scavenging capacity in situ without requiring the unstable free acid in the formulation
CIR Expert Panel concluded safe as used in cosmetic formulations:51-111, 2005; PMID 16154915; December 2025 QRT Finding=S) — group assessment with no sensitization risk identified, no carcinogenic concern, and no genotoxicity attributable to the ascorbate moiety
Comparable anti-wrinkle efficacy to ascorbic acid: 5% SAP and 5% L-ascorbic acid emulgels produced statistically equivalent reductions in crow's-feet wrinkle depth and elasticity improvements over the test period, with SAP offering formulation stability advantages (Mohammadi et al. 2021, PMID 32383548)
Functions as a prodrug: requires cleavage by endogenous skin phosphatases to release active ascorbic acid; conversion efficiency depends on enzyme activity and skin condition, and percutaneous absorption of the highly polar trisodium salt is intrinsically lower than less-polar derivatives like tetrahexyldecyl ascorbate
Like ascorbic acid, may exhibit prooxidant activity in the presence of free transition metal ions (Fe2+, Cu2+) via Fenton-type chemistry once dephosphorylated; not a concern at typical cosmetic use levels per the CIR 2005 review
Phosphorylation does not eliminate all oxidative degradation: aqueous SAP formulations still benefit from added antioxidant stabilizers per stability studies (Mohammadi et al., 2021), and turn yellow-to-brown over extended storage at higher temperatures
CIR 2005 group assessment is the primary regulatory record; no separate or updated CIR final report on SAP exists, and the 2022 ascorbyl ethers/esters reassessment:57S-75S) covered different derivatives — SAP was not re-reviewed in that report
CIR Quick Reference Table (December 2025) — Sodium Ascorbyl Phosphate row: Finding=S, Citation=IJT 24(Suppl. 2):51-111, 2005
“Sodium Ascorbyl Phosphate | S | IJT 24(Suppl. 2):51-111, 2005”— QRT-Update-Dec2025.pdf, p. 410