AMSTERDAM -- The Dutch government said Friday it would move to classify high-potency marijuana alongside hard drugs such as cocaine and ecstasy, the latest step in the country's ongoing reversal of its famed tolerance policies.
The decision means most of the cannabis now sold in the Netherlands' weed cafes would have to be replaced by milder variants. But skeptics said the move would be difficult to enforce, and that it could simply lead many users to smoke more of the less potent weed.
Possession of marijuana is technically illegal in the Netherlands, but police do not prosecute people for possession of small amounts, and it is sold openly in designated cafes. Growers are routinely prosecuted if caught.
Economic Affairs Minister Maxime Verhagen said weed containing more than 15 percent of its main active chemical, THC, is so much stronger than what was common a generation ago that it should be considered a different drug entirely.
The high potency weed has "played a role in increasing public health damage," he said at a press conference in The Hague.
The Cabinet has not said when it will begin enforcing the rule.
Jeffrey Parsons, a psychologist at Hunter College in New York who studies addiction, said the policy may not have the benefits the government is hoping for.
"If it encourages smoking an increased amount of low-concentration THC weed, it is likely to actually cause more harm than good," he said, citing the potential lung damage and cancer-causing effects of extra inhalation.
The Dutch Justice Ministry said Friday it was up to cafes to regulate their own products and police will seize random samples for testing.
But Gerrit-Jan ten Bloomendal, spokesman for the Platform of Cannabis Businesses in the Netherlands, said implementing the plan would be difficult "if not impossible."
"How are we going to know whether a given batch exceeds 15 percent THC? For that matter, how would health inspectors know?" he said. He predicted a black market will develop for highly potent weed.
The ongoing Dutch crackdown on marijuana is part of a decade-long rethink of liberalism in general that has seen a third of the windows in Amsterdam's famed prostitution district shuttered and led the Netherlands to adopt some of the toughest immigration rules in Europe.
The number of licensed marijuana cafes has been reduced, and earlier this year the government announced plans to ban tourists from buying weed. That has been resisted by the city of Amsterdam, where the marijuana cafes known euphemistically as "coffee shops" are a major tourist draw.
Marjan Heuving of the Netherlands' Trimbos Institute, which studies mental health and addiction, said there is a growing body of evidence that THC causes mental illnesses.
She said it stands to reason "the more THC the body takes in, the more the impact." But it has not been demonstrated scientifically that high THC weed is worse for mental health, she said.
Parsons of Hunter College said it remains difficult to be sure whether marijuana causes mental problems or whether people predisposed to, say, depression seek it out as a form of self-medication.
The Trimbos Institute says the average amount of THC in Dutch marijuana is currently around 17.8 percent. It has been declining since 2004 after increasing steadily from 4 percent or so in the 1970s.
By comparison, in the United States the average level of THC in marijuana is around 10 percent and rising, according to the last measure released by the Office of National Drug Control Policy in 2009.
Heuving agreed with Ten Bloomendal that determining THC levels outside of a laboratory setting would prove difficult, as exact content varies widely from batch to batch and even within a single plant.
"I don't know of any home test," she said. "How this is going to work in practical terms, I have no idea."