Richard Vogel, Associated Press
In this Tuesday Jan. 26, 2010 file photo, a pedestrian walks past a marijuana leaf neon sign advertising a medical marijuana provider along a street in the Sherman Oaks section of Los Angeles, Calif.

Opponents of the movement to allow marijuana use for medical purposes have long warned it is the foot in the door toward what supporters really want — full acceptance of recreational marijuana use. Those warnings are gradually coming true.

Later this week, Washington state is expected to certify petition signatures to place a measure on November's ballot legalizing marijuana for recreational purposes. Colorado officials may do the same with an initiative petition there soon. Both states already allow medical marijuana, as do 14 others and the District of Columbia. Supporters of the initiatives say they have a responsible approach that restricts usage to adults. Washington would allow sales through only those outlets licensed by the state, which would control production. Drunken driving laws would be changed to include limits on the blood content of THC, the main psychoactive chemical in marijuana.

But there really is no responsible way to expand the universe of harmful substances that are acceptable by law. While there are legitimate arguments for changing the focus of the criminal justice system to one of treatment, rather than imprisonment, for users, there would be no public benefit in making the drug legal. Marijuana is a mind-altering and addictive substance that is detrimental to health.

Even the arguments that legal marijuana would undercut and destroy the lucrative illegal drug trade should be tempered by a dose of reality. A tightly controlled marijuana market still would encourage drug traffickers to cultivate and sell a type of marijuana that is more potent than the official brand. Meanwhile, the idea that sanctioning a drug would somehow reduce the rate of usage, as some claim, is fuzzy thinking, at best. For several reasons, including history and culture, the marijuana trade is not analogous to alcohol during prohibition.

Fortunately, the White House has been consistent in its policy against the legalization of marijuana. In a statement late last year, the Obama administration seemed to indicate it is prepared to continue the long-standing policy of not granting immunity from federal drug prosecution because of contradictory state laws.

At the time, Obama's director of the Office of National Drug Control Policy said, "...legalizing marijuana would not provide the answer to any of the health, social, youth education, criminal justice, and community quality of life challenges associated with drug use." He also mentioned the drug's connection to respiratory disease, cognitive impairment and addiction.

Supporters have been buoyed by a recent Gallup poll that showed 50 percent of Americans supporting legalization, a record level of support. That is the opposite of what appears to be happening in Europe, where support is dwindling despite some countries' experience with permissive laws.

California voters rejected a similar legalization measure in 2010. They may have been influenced by the strong opposition of the American Society for Addiction Medicine, as well as by common sense. We hope similar thinking prevails in Washington and Colorado.