Morris you asked about meds (specifically ADs) and bf: very few meds can be actively tested on bf women. However, the studies that are done assess for safety in one or more ways:
i) the pharmacological make-up of the drug is assessed for how readily it can travel through the blood to the milk - the molecular structure of some is that it does not get into the milk
ii) what might happen to the drug if it reaches the milk and then to the baby's tum - if stomach acids destroy the active ingredient it should be safe
iii) what proportion of the drug gets into the milk, if any - if it's tiny, it will be safer than if it's large
iv)how vulnerable the baby is - tiny babies = more vulnerable than toddlers
v)how long the drug stays active before 'dying' - if it's half an hour, say, it's easy for the mother to time her doses
vi) looking at what happens to milk, and to babies, where mothers have taken the drug because they were prescribed it (ADs are relatively commonly prescribed for bf mothers, so there are several studies on these)
That's how the safety of ADs and bf can be assessed.