I haven’t worked on any clinical trials or clinical research. But I came across this website that graphically describes the reasons for clinical trial failures from 2013 to 2015. It seems like “efficacy” is the most common cause of failure in Phase II and Phase III trials. There are more operational and commercial reasons for failure in Phase III trials than in Phase II. However, trials fail for safety and strategy in Phase II than in Phase III trials.

I wish I was able to read this full article. It would be interesting to know a couple of specific examples or understand what the scope of efficacy means for the different therapeutic areas. Check out the link below to see the pie charts.

http://www.nature.com/nrd/journal/v15/n12/fig_tab/nrd.2016.184_F1.html?foxtrotcallback=true