Oxiracetam is a medication and nootropic which has been marketed popularly as a diet supplement, purported to have cognition and memory enhancing effects. It has no known lethal dosage in humans, and there are no known reports of fatalities documented in literature. Like other racetams, it acts as a positive allosteric modulator of the AMPA receptors, and also the mGlu receptors. The S-isomer is found to be the major active ingredient in formulations involving oxiracetam, with the R-isomer being largely inactive in clinical trials.
One multicenter, double-blind, placebo controlled clinical trial (conducted in 1992) found oxiracetam to significantly improve neurological test scores in adults with primary degenerative, multi-infarct or mixed dementia. 800 mg administered twice daily was able to increase test scores assessing literacy, nonverbal memory, reaction times, long term/short term memory, and problem solving abilities. Tests during the course of treatment were conducted over the course of 6 to 12 weeks. 
This compound is not for human consumption, and its sale is strictly intended for research purposes only. Available as the racemic of the S and R isomer.
 - Bottini, G., et al. “Oxiracetam in Dementia: a Double-Blind, Placebo-Controlled Study.” Acta Neurologica Scandinavica, vol. 86, no. 3, 1992, pp. 237–241., doi:10.1111/j.1600-0404.1992.tb05077.x.