There's 2 possible outcomes imho. The Ornarans last transports fall apart, because they're too doped to keep them functional... shipments stop. Panic escalates that likely costs millions of lives. Whatever time it takes for the withdrawal to last, some vast majority survive its subsidence, whereupon they then learn they weren't fatally infected (Which is why Picard should've just told them anyway, to maybe save lives)
They realize the Brekkians have been enslaving them with addictive drugs, & while currently too decimated to act on that grudge, they could, once clear headed, be able to pull themselves together enough to retaliate... violently maybe
The Brekkians, who are way worse off long term, have their entire global economy collapse, unless they can find some other equivalent buyer for the bloated supply of their sole export, which is very unlikely, because there ultimately is no realistic substitute for an entire race of people who you are holding hostage with your product, short of another one just like them.
Once you lose your slaves, they're usually not getting replaced at that level. They could find some buyers for their narcotic, just like any other drug dealer, but it's never going to replace what they had, & it'll be ages before they can build a functioning society without what they've lost, or they might be utterly doomed to self-destruction.
OR... realizing what will come, the Brekkians find an alternate solution to move their shipments to the Ornarans somehow, who, thanks to Picard, don't know they can kick the addiction, & the status quo returns, with a possible third party opting to profit off the transportation, if the Brekkians can't manage it themselves... at least until the Ornarans are completely useless from drug addiction, & this nonsense implodes some other way
Ultimately the Ornarans will recover just fine, because they've been the go-getters this whole time, & are only falling apart because they're suffering from addiction, once they aren't, they will thrive. The Brekkians OTOH, have willfully diminished their futures for a life of luxury, & someday will pay the price