Where to start!? Leaches, blood letting, pretty much everything doctors did in the pre-modern period…
Thalidomide and birth defects, of course. But unforeseen.
X-rays for pregnancy monitoring. Took years before people paid attention to the alarms. X-rays were also used for tinea capitis – not just brain tumours, strokes and ischaemic heart disease about 30% higher too.
Ribavirin (via SPAG machines) for RSV. Not harmful, just useless and expensive.
Iron supplements for preterm babies – increased sepsis.