Case in point is in regard to hormone replacement therapy (HRT) for menopausal women.It has not been easy to get that right.
The stylized facts of the history of that effort briefly are:
Use HRT widely as heart disease will be prevented plus the usual listing of such things as better skin,lessened hot flashes,improved mentation and all of the advantages of being younger rather than older.
Give HRT to almost no one as actually it increases not decreases incidence of heart disease.Yeah we got the sign wrong.And then there was the issue of increased blood clots and cancer risk.
Then a study demonstrates decreased incidence of clinical heart disease if the HRT is given early after the onset of menopause.The earlier data that demonstrated increased heart disease was derived from a study of older women who received HRT later after the onset of menopause. This suggests that a major determinant of outcome is timing.
See here for the more recent study on HRT in which women received HRT soon after the onset of menopause which demonstrated a decrease in the incidence of clinical heart disease.
Preventive medicine is not rocket science. The rocket scientists know with impressive accuracy where the rocket will come down as the law of physics applicable to that application are pretty well worked out. Preventive medicine is much harder.
addendum: See this essay on the arrogance of preventive medicine by Dr. David Sackett. Preventative ( the once incorrect term is now acceptable) medicine, of course, is a cornerstone of "population medicine" with its arrogance magnified.