Chalk one up to the law of unintended consequences.

Since well before I-502, Washington’s legal marijuana initiative, one of the major arguments for legalizing pot has been that driving down prices would dry up the violent black market for marijuana. Dan Savage, among others, was especially vocal in touting this not only as a justification, but as an obligation – that our drug prohibition was creating a black market so lucrative that Central American drug cartels could use the profits to take over entire countries.

They were right. Mostly.

According to WaPo, that drying up is exactly half of what’s happening. The price of export pot in Mexico has dropped by a whopping 75 percent – from about $100 to about $25 per kilo – and farmers there are indeed not bothering to plant pot. One farmer apparently said, “it’s not worth it anymore. I wish the Americans would stop it with this legalization.”

The other half is the reality, that as usual, we failed to predict. Those farmers have bills to pay, and legalization or not, Spring came as usual this year. They needed something to put in the ground, and you can imagine that the profit margin on corn hasn’t gotten any more attractive.

So they planted heroin poppies. Remember those stories about the heroin epidemic gripping towns big and small over the last few months? Apparently, it’s coming overwhelmingly from Central America, where it opium poppies have replaced marijuana as the cash crop for those already in the business of supplying the cartels.

That the people producing one thing made absurdly valuable by our drug prohibition have now switched to another, using all the infrastructure already in place, shouldn’t come as a surprise. If anything, it validates the original argument – the pot market really does appear to be drying up.