Most Americans are paying too much for marijuana. I’m not referring to people who smoke it—using the drug generally costs about as much as using alcohol. Marijuana is unaffordable for the rest of America because billions are wasted on misdirected drug education and distracted law enforcement, and we also fail to tax the large underground economy that supplies cannabis.

On Monday, the New Jersey legislature passed a bill legalizing marijuana for a short list of medical uses. Outgoing Democratic Gov. Jon Corzine says he will sign it into law. This is a positive step, as cannabis has several unique medical applications. But the debate over medical marijuana has obscured the larger issue of pot prohibition.

As a psychiatrist, I treat individuals who often suffer from devastating substance abuse. Over many years of dealing with my patients’ problems, I have come to realize that we are wasting precious resources on the fight against marijuana, which more closely resembles legal recreational drugs than illegal ones. My conscience compels me to support a comprehensive and nationwide decriminalization of marijuana.

Prohibition did decrease alcoholism and alcohol consumption in the 1920s. However, the resulting rise of violent organized crime and the loss of tax revenue were untenable and led to the repeal of Prohibition. By analogy, while the broad decriminalization of marijuana will likely reduce the societal and economic costs of pot prohibition, it could lead to more use and abuse.

The risks of marijuana use are mild compared to those of heroin, ecstasy and other illegal drugs, but the drug is not harmless. A small number of my patients cannot tolerate any use without serious impact on underlying disorders. Others become daily, heavy smokers, manifesting psychological if not physiological dependence. While most of my patients appear to suffer no ill effects from occasional use, the drug makes my work more difficult with certain individuals.

So why do I support decriminalization? First, marijuana prohibition doesn’t prevent widespread use of the drug, although it does clog our legal system with a small percentage of users and dealers unlucky enough to be prosecuted. More to the point, legal cannabis would never become the societal problem that alcohol already is.

In most of my substance-abuse patients, I am far more concerned about their consumption of booze than pot. Alcohol frequently induces violent or dangerous behavior and often-irreversible physiological dependence; marijuana does neither. Chronic use of cannabis raises the risk of lung cancer, weight gain, and lingering cognitive changes—but chronic use of alcohol can cause pancreatitis, cirrhosis and permanent dementia. In healthy but reckless teens and young adults, it is frighteningly easy to consume a lethal dose of alcohol, but it is almost impossible to do so with marijuana. Further, compared with cannabis, alcohol can cause severe impairment of judgment, which results in greater concurrent use of hard drugs.

Many believe marijuana is a gateway drug—perhaps not so harmful in itself but one that leads to the use of more serious drugs. That is not borne out in practice, except that the illegal purchase of cannabis often exposes consumers to profit-minded dealers who push the hard stuff. In this way, the gateway argument is one in favor of decriminalization. If marijuana were purchased at liquor stores rather than on street corners where heroin and crack are also sold, there would likely be a decrease in the use of more serious drugs.

The nation badly needs the revenue of a “sin tax” on marijuana, akin to alcohol and tobacco taxes. Our government could also save money by ending its battle against marijuana in the drug war and redirecting funds to proactive drug education and substance-abuse treatment. Hyperbolic rants about the evils of marijuana could give way to realistic public education about the drug’s true risks, such as driving under the influence.

Our nation can acknowledge the dangers of cigarettes, alcohol and marijuana while still permitting their use. The only logically and morally consistent argument for marijuana prohibition necessitates the criminalization of all harmful recreational drugs, including alcohol, nicotine and caffeine. We can agree that such an infringement on personal freedoms is as impractical as it is un-American. The time has come to accept that our nation’s attitude toward marijuana has been misguided for generations and that the only rational approach to cannabis is to legalize, regulate and tax it.

Dr. Nathan, a psychiatrist in Princeton, N.J., is a clinical assistant professor at Robert Wood Johnson Medical School.