The apothecary guild gained most of its current power thanks to a very profitable business: get healed of any illness for an amount of money.
Thanks to its reservoir full of a strange and voltaile liquid that is sprayed to the user, it is possible to instaneously recover from a wide range of injuries including broken bones, pneumonia or knife stabs.
You can find their machines on every corner of the city, specially the rich districts. Thankfully, these machines are not specially though and can be 'sabotaged' quite easily.
Izzy can use them to recover health along all the game:
