in vivo like a b-vitamin and at higher doses like plant-derived nutrients, such as curcumin, sulforaphane, resveratrol, and other nutritional substances that induce phase ii detoxification enzymes, thus acting as cytoprotective agents. this stress response indirectly improves the antioxidant capacity of the cell. the (s)-enantiomer of la was shown to be toxic when administered to thiamine-deficient rats. several studies have demonstrated that sla either has lower activity than rla or interferes with the specific effects of rla by competitive inhibition. uses r/s-la and rla are widely available as over-the-counter nutritional supplements in the united states in the form of capsules, tablets, and aqueous liquids, and have been marketed as antioxidants. although the body can synthesize la, it can also be absorbed from the diet. dietary supplementation in doses from 200–600 mg is likely to provide up to 1000 times the amount available from a regular diet. gastrointestinal absorption is variable and decreases with the use of food. it is therefore recommended that dietary la be taken 30–60 minutes before or at least 120 minutes after a meal. maximum blood levels of la are achieved 30–60 minutes after dietary supplementation, and it is thought to be largely metabolized in the liver. in germany, la is approved as a drug for the treatment of diabetic neuropathy since 1966 and is available as a non-prescription pharmaceutical. clinical research according to the american cancer society as of 2013, "there is no reliable scientific evidence at this time that lipoic acid prevents the development or spread of cancer". as of 2015, intravenously administered ala is unapproved anywhere in the world except germany for diabetic neuropathy, but has been proven reasonably safe and effective in four clinical trials; however another large trial over four years found no difference from placebo. as of 2012, there was no good evidence alpha lipoic acid helps people with mitochondrial disorders. a 2018 review recommended ala as an anti-obesity supplement with low dosage (< 600 mg/day) for a short period of time (<10 weeks); however, it is too expensive to be practical as a complementary therapy for obesity. other lipoic acids β-lipoic acid is a thiosulfinate of α-lipoic acid