Stable, gentler forms of vitamin C that don't brown or sting, but must be converted in skin and have far less evidence than pure ascorbic acid.
Because L-ascorbic acid is fragile and needs a very acidic formula, chemists attach it to a sugar (ascorbyl glucoside), a phosphate (sodium or magnesium ascorbyl phosphate) or a fatty acid (ascorbyl palmitate). These keep longer, work at a skin-friendly pH and rarely sting.
The trade-off is delivery: enzymes in skin must cut them back to ascorbic acid, and how much gets through is uncertain. In the pig-skin study that set the rules for vitamin C serums, magnesium ascorbyl phosphate and ascorbyl palmitate did not raise skin vitamin C levels at all. Ascorbyl palmitate is mostly used to protect a formula's oils.
Evidence is mostly lab work plus a few small clinical studies, often run by ingredient makers and without placebo groups: a 10% magnesium ascorbyl phosphate cream lightened spots in about half of patients with melasma or age spots, and a 5% sodium ascorbyl phosphate lotion was reported to help acne in an open study. There are almost no head-to-head trials against L-ascorbic acid.
L-ascorbic acid entered pig skin only when formulated below pH 3.5, with absorption maximal at 20%; skin levels saturated after three daily applications and fell with a half-life of about 4 days. Magnesium ascorbyl phosphate, ascorbyl palmitate and dehydroascorbic acid did not raise skin vitamin C levels.
Magnesium ascorbyl phosphate inhibited melanin formation in lab tests and reached the epidermis; a 10% cream produced significant lightening in 19 of 34 patients with melasma or age spots and in 3 of 25 with normal skin.
From the ingredient's manufacturer: 1% sodium ascorbyl phosphate killed acne bacteria in the lab, a formula with it reduced UVA-induced sebum oxidation in 20 people, and a 5% lotion was rated excellent or good in 77% of 60 people with acne in an open 12-week study.