Penoxycyline is a medical drug in RimWorld which was introduced in Alpha 16, and succeeds Malari-Block. Unlike its predecessor, Penoxycyline blocks against 3 diseases: malaria, sleeping sickness, and the dreaded plague - whereas Malari-block only blocked against malaria.
Like Malari-Block; Penoxycyline can either be purchased from traders, or crafted at a drug lab - but for a mere 3 neutroamine as opposed to the 5 which Malari-block took. However, in order to make Penoxycyline, you need to have penoxycyline production researched, which in turn requires drug production, and Neutroamine has to either be purchased from traders, or found in cargo pods.
Penoxycyline can be administered if one has one of the diseases it protects against, and doing so will boost the immunity to 61% if it's lower than that.