Glutamine is an amino acid used mainly as a skin-conditioning/humectant-support ingredient, typically at low concentrations, and it is generally well tolerated because it is not inherently acidic, solvent-like, or strongly reactive. Clinical irritation signals for glutamine itself are uncommon in patch-testing literature compared with fragrances, preservatives, and actives, but compromised eczema skin can still sting to otherwise mild amino-acid solutions depending on formula osmolality and pH. To avoid underestimating risk in highly sensitive populations while reflecting its overall gentle profile, it is scored as very gentle. Safety Notes: In commercial skincare, glutamine is most often used as a skin-conditioning/amino-acid support ingredient in multi-amino-acid blends and barrier-support serums/creams, where it can appear at low “label presence” levels around ~0.01–0.1% in leave-on products. Higher-strength consumer-available formulas (typically leave-on serums or creams marketed for barrier repair, post-procedure comfort, or microbiome/skin recovery) have been observed using glutamine around ~0.5–2% as a dedicated active within amino-acid/NMF complexes. Above ~2% is uncommon in mass-market OTC due to diminishing formulation benefit, solubility/osmolality considerations, and the tendency for brands to shift to broader amino-acid systems rather than pushing single-amino-acid load.