[WhiteSage]

You have to fix the hypotheses to test before you begin the experiment, as otherwise you will stumble upon a lot of spurious correlations. That your birthday is the same day than someone's in your class/work group is a rare event, that two people have the same birthday in your class/work group, not at all uncommon. This is the birthday paradox. In the same spirit, with a drug you'll always find groups where it works because of spurious correlations, specially if you don't fix beforehand the group where you want to test it.

[derefr]

Run one trial to find correlations with specific groups, then run another trial under the hypothesis that it works for that group.

[nradov]

Well sure but that costs a fortune. Drug development is already extremely expensive.

[ncmncm]

That is a good idea, but it generates accusations of special pleading. Medical science is plagued with many kinds of fetish, and medical doctors are not generally obliged to actually understand science.