A post-hoc analysis involves looking at the data after a study has been concluded, and trying to find patterns that were not primary objectives of the study. In other words, all analyses that were not pre-planned and were conducted as 'additional' analyses after completing the experiment are considered to be post-hoc analyses. A post-hoc study is conducted using data that has already been collected. Using this data, the researcher conducts new analyses for new objectives, which were not planned before the experiment. Thus, analyses of pooled data from previously conducted trials could be a form of post hoc study.
I don’t think a clinical trial conducted on specific efficacy or safety parameter from previous trials can be a post-hoc study. Efficacy and safety would generally be primary objectives for a trial, that is, these would be pre-planned, whereas, a post-hoc study should include only analyses conducted after the completion of the experiment.