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MANY baby boomers and their children still believe the folklore of the 1960s and ’70s that marijuana can’t be addictive, because it’s just an herb. But several decades of research clearly show that what we believed 40 years ago is wrong. Marijuana addiction is common.

About 9 percent of people who smoke marijuana even once become addicted to it, and that figure approximately doubles when people begin using the drug as adolescents.

To prevent addiction from spreading, Washington voters should not legalize marijuana and reject Initiative 502 on the Nov. 6 ballot.

Marijuana addiction statistics are similar to the percentage of people who become alcoholics. Rates of marijuana dependence rise to 20 to 30 percent when people use it at least five times, and 35 to 40 percent for those who use marijuana daily. Marijuana dependence is the most common type of drug dependence in the United States, besides alcohol and tobacco.

As doctors who have practiced addiction medicine for decades, we’re seeing unprecedented levels of marijuana addiction today. Treatment admissions for marijuana-use disorders have dramatically increased, accounting for 18 percent of admissions in 2010, higher than for heroin, methamphetamine, cocaine and prescription painkillers. We attribute rising marijuana addiction to increases in use and the astounding rise in marijuana potency — more than 760 percent higher than in the 1960s.

The neurobiology of marijuana dependence is well understood through recent research. But the symptoms of marijuana dependence are the most convincing evidence. The main indicators of dependence are withdrawal symptoms, unsuccessful attempts to control use, increased tolerance and using larger amounts of the drug or for a longer time than intended. All of these have been measured in people with marijuana dependence. Laboratory studies of brain responses reveal neurochemical changes from marijuana withdrawal similar to other dependence-producing drugs.

Upon abstinence, daily marijuana smokers experience withdrawal symptoms including anxiety, depression, irritability, restlessness, sleep difficulty, strange dreams, anger and aggression — the same symptoms experienced when people stop using heroin and cocaine.

The American Society of Addiction Medicine recently issued a white paper outlining our position against marijuana legalization. Our principal objection is that legalization will increase the disease of addiction. We must oppose a massive social experiment that would increase the disease we strive each day to reduce.

A RAND Corporation study of California’s 2010 legalization ballot measure found that marijuana use would substantially increase.

Legalization proponents say that addiction and other health problems from legal marijuana could be addressed through heavy taxation to pay for prevention and treatment and regulation to keep it away from kids. That hasn’t worked with our two legal drugs — alcohol and tobacco — because their powerful industries kill attempts at taxation and regulation.

Commercial marijuana would quickly become a highly profitable industry. What powerful industry in this country simply allows itself to be heavily taxed and regulated?

We often hear legalization proponents argue that alcohol and tobacco cause more addiction and other health problems than marijuana. This is a curious rationale for legalization. If an illegal drug is not causing as many health problems as legal drugs, we should not legalize it so it can cause more health problems. Instead, we should try to reduce the use of all drugs and the health problems they cause.

Addiction isn’t the only health problem caused by marijuana use. Recent research also shows that marijuana use is linked to dangerous drugged driving; neurocognitive damage, particularly in young people; and increased rates of anxiety, depression and schizophrenia. Marijuana smoke contains more carcinogens than tobacco.

We know it’s hard to admit when we’re wrong, but many baby boomers got marijuana wrong, and they’ve passed their denial of marijuana’s dangers on to their children.

Dr. Robert DuPont, left, is a former White House drug chief and former director of the National Institute on Drug Abuse. Dr. Andrea Barthwell is a former deputy director for demand reduction at the Office of National Drug Control Policy.