Was it just that the penicillin salts they were using at the time had low bioavailability? That is, do we now have ways of administering penicillin or other ant

Was it just that the penicillin salts they were using at the time had low bioavailability? That is, do we now have ways of administering penicillin or other antibiotics such that less of the drug is expelled unused, or do we simply produce a high enough volume of antibiotic drugs that it doesn’t matter?