The Czech Republic should be proud. Its customs just seized $37 million in heroin (paywall), the country’s largest drug bust ever. The shipment is mammoth by any standards, but especially Czech standards—the 182 kilograms (401 pounds) could be equivalent to an entire year’s worth of heroin use in the Czech Republic.

And yet, the government isn’t taking much of a victory lap. Vlastimil Necas, a member of the Czech government’s drug monitoring center, told the Wall Street Journal that it was a success—but he added a pretty hefty caveat.

“…despite this size of the seizure, Mr. Necas said it likely won’t lead to a decrease in drug use, but rather may exacerbate the current shortage of heroin on Europe’s black market, drive up prices and could have the adverse effect of leading to a minor increase in crime as addicts look for ways to generate cash to feed their addiction.”

One of the problems with seizing a shipment as large as the one Czech authorities just did is that it can shock a sensitive underground system. There’s historical precedent for that. A 2010 drought in leading poppy seed producer Afghanistan halved its heroin production, causing significant black market price increases in the European Union.

Those price hikes open the door for increasing violence, as Necas pointed out. But there are other, more complicated consequences too. Drug droughts—heroin ones specifically—can affect the purity, or strength, of the drugs being sold on the black market. That’s a problem, because it makes it difficult for local users to gauge dosages. When addicts are used to less potent batches, stronger ones can be deadly. In 2011, on the heels of an extended period of heroin shortage, Britain saw a rise in the number of deaths from heroin overdoses, after more potent stuff entered the country.

And then there’s the threat that in the face of higher prices, local drug users turn away from the likes of heroin, and toward other, cheaper, synthetic drugs. A 2013 report from the European Monitoring Center for Drugs and Drug Addiction noted exactly that (pdf p. 13). Normally, shelving heroin would be a good thing, but not when alternatives have scarier side effects and higher overdose rates.

None of this is to say the Czech government doesn’t see the bust as a success—it does, and should. But big drug busts carry the potential for big headaches.