Quid pro quo and poisons for medicines

Meaning

Via uses Quid pro quo for giving one thing in place of another, then specifies the grave substitution at issue with Venena pro medicamentis: poisons instead of medicines. The context is an accusation against failed alchemists who claim medical authority without understanding serious illness. It is not the modern transactional sense of an exchange of favours. The narrator says such people cause deaths and must answer to God, while secular authorities should restrain them. The aligned English makes the substitution explicit without treating it as an approved therapeutic practice. The accusation remains the narrator’s, not an independent finding about every occupation he names.