The party can bribe a dock agent to see a list of ships that arrived about a month ago. The ship identified by the dealer came from a small island. They will either be sent or feel obligated to put a stop to this operation. A ship can be hired to take them there. (DM's can use this as an opportunity for a sea battle with one or more of the drug-lords ships.)
Drax is made from the dragon flower which is only found on Dragon Island. The drug-lords know its effects, and hope to use it to build income and at the same time wipe out the mainland population.
The drug-lords are non-human but employ human operatives for distributing the drug. DM's are free to use whatever creatures would best fit the party and the campaign. Originally, this adventure was used for a low-to-mid level party and had ogre magi as the head of the operation with a number of humanoids (mostly hobgoblins) in their employment. For higher level parties, the head drug-lord could easily be a dragon. (This is Dragon Island after all.)
The dragon flowers can be wiped out using a variety of methods. However, completely eliminating this species may be frowned on by any druids in the area, as the flower does have other beneficial uses and is quite beautiful.