Glucosamine

Glucosamine is typically used in leave-on skincare at low percentages (often ~1–5%) as a humectant/skin-conditioning and tone-support ingredient, and it is generally well tolerated in patch testing and clinical use. However, in highly reactive or eczematous skin it can still sting or irritate—especially in compromised barriers or when combined with other actives—so I rate it as gentle but not “very gentle/inert” for patient safety. Safety Notes: In mass-market and prestige leave-on serums/creams, glucosamine is commonly present at low supportive levels around 0.1–1% (often paired with niacinamide for tone/brightening claims), and it also appears at similar or slightly lower levels in some rinse-off cleansers. High-strength OTC brightening/spot-correcting leave-on treatments have been marketed in the ~2–5% range, with ~5% representing the upper end typically seen due to solubility, pH/salt form (HCl vs sulfate), and sensory/stability constraints rather than regulatory limits.