Actually, trials are sometimes stopped early for "efficacy." This is done on the grounds that--past some point--incremental gains in statistical certainty are n

Actually, trials are sometimes stopped early for "efficacy." This is done on the grounds that--past some point--incremental gains in statistical certainty are not worth depriving the subjects in the control arms of the drugs' benefits. For example, this happened in the PARADIGM-HF trial in 2014: http://www.forbes.com/sites/larryhusten/2014/03/31/novartis-trial-was-stopped-early-because-of-a-significant-drop-in-cardiovascular-mortality/#a6d596075aca http://www.forbes.com/sites/larryhusten/2014/03/31/novartis-... This isn't totally uncontroversial (e.g., https://www.ncbi.nlm.nih.gov/pubmed/18226746 https://www.ncbi.nlm.nih.gov/pubmed/18226746 ) but it's not obvious to me how to balance statistical and ethical concerns. As for the animal studies, it sometimes happens. Someone up-thread mentioned SuperMAB, a potential drug which seemed safe in monkeys. However, when first tested in humans (using a dose 500x lower than the one administered to the monkeys), it generated a cytokine storm that put all six test subjects in the ICU. Here's a fairly decent summary: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2964774/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2964774/