Lactic Acid, L- (L-lactic acid, L-lactate) is the levorotatory isomer of lactic acid, the biologically active isoform in humans. Lactic acid or lactate is produced during fermentation from pyruvate by lactate dehydrogenase. Lactate dehydrogenase is isomer-specific so that production and metabolism of D-lactate require D-LDH and L-lactate requires L-LDH. Mammalian cells only contain L-LDH so that in humans the lactate produced is almost exclusively L-lactate. Carbohydrate-fermenting bacterial species (e.g. lactobacillus spp) have by contrast both enzymes and therefore the capacity to produce both D-lactate and L-lactate. L-lactic acid is the primary agonist of hydroxycarboxylic acid receptor 1 (HCA1), which is a Gi/o-coupled G protein-coupled receptor (GPCR). Poly-L-lactic acid is an absorbable, semi-permanent, injectable implant that can be used to restore volume and stimulate collagen formation gradually. It is FDA-approved for the correction of facial fat loss associated with antiretroviral therapy-induced lipoatrophy in HIV patients. It is FDA-approved for use in immunocompetent people for the correction of nasolabial fold deficiencies and other facial wrinkles. It has been used off-label to enhance the cheeks, hands, neck, thighs, gluteal enhancement and chest wall deformities, such as pectus excavatum or thoracic deformities secondary to surgical procedures.