[cSoze]

I mean.

(1) would absolutely not be continued, obviously.

(2) would be continued to completion.

In science one is maximally conservative with positive results. Negative results can be accepted rather quickly but positive results need to be maximally investigated. False positives are unacceptable, false negatives are often less damaging.

I'd question how scenario #1 made it through animal trials preceding experiments in humans.

[mattkrause]

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-...

This isn't totally uncontroversial (e.g., 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/