For Michele Leonhart, the administrator of the Drug Enforcement Administration, there is no difference between the health effects of marijuana and those of any other illegal drug. “All illegal drugs are bad for people,” she told Congress in 2012, refusing to say whether crack, methamphetamines or prescription painkillers are more addictive or physically harmful than marijuana.

Her testimony neatly illustrates the vast gap between antiquated federal law enforcement policies and the clear consensus of science that marijuana is far less harmful to human health than most other banned drugs and is less dangerous than the highly addictive but perfectly legal substances known as alcohol and tobacco. Marijuana cannot lead to a fatal overdose. There is little evidence that it causes cancer. Its addictive properties, while present, are low, and the myth that it leads users to more powerful drugs has long since been disproved.

That doesn’t mean marijuana is harmless; in fact, the potency of current strains may shock those who haven’t tried it for decades, particularly when ingested as food. It can produce a serious dependency, and constant use would interfere with job and school performance. It needs to be kept out of the hands of minors. But, on balance, its downsides are not reasons to impose criminal penalties on its possession, particularly not in a society that permits nicotine use and celebrates drinking.

Marijuana’s negative health effects are arguments for the same strong regulation that has been effective in curbing abuse of legal substances. Science and government have learned a great deal, for example, about how to keep alcohol out of the hands of minors. Mandatory underage drinking laws and effective marketing campaigns have reduced underage alcohol use to 24.8 percent in 2011, compared with 33.4 percent in 1991. Cigarette use among high school students is at its lowest point ever, largely thanks to tobacco taxes and growing municipal smoking limits. There is already some early evidence that regulation would also help combat teen marijuana use, which fell after Colorado began broadly regulating medical marijuana in 2010.

Comparing the Dangers As with other recreational substances, marijuana’s health effects depend on the frequency of use, the potency and amount of marijuana consumed, and the age of the consumer. Casual use by adults poses little or no risk for healthy people. Its effects are mostly euphoric and mild, whereas alcohol turns some drinkers into barroom brawlers, domestic abusers or maniacs behind the wheel.

An independent scientific committee in Britain compared 20 drugs in 2010 for the harms they caused to individual users and to society as a whole through crime, family breakdown, absenteeism, and other social ills. Adding up all the damage, the panel estimated that alcohol was the most harmful drug, followed by heroin and crack cocaine. Marijuana ranked eighth, having slightly more than one-fourth the harm of alcohol.

Federal scientists say that the damage caused by alcohol and tobacco is higher because they are legally available; if marijuana were legally and easily obtainable, they say, the number of people suffering harm would rise. However, a 1995 study for the World Health Organization concluded that even if usage of marijuana increased to the levels of alcohol and tobacco, it would be unlikely to produce public health effects approaching those of alcohol and tobacco in Western societies.

Most of the risks of marijuana use are “small to moderate in size,” the study said. “In aggregate, they are unlikely to produce public health problems comparable in scale to those currently produced by alcohol and tobacco.”

While tobacco causes cancer, and alcohol abuse can lead to cirrhosis, no clear causal connection between marijuana and a deadly disease has been made. Experts at the National Institute on Drug Abuse, the scientific arm of the federal anti-drug campaign, published a review of the adverse health effects of marijuana in June that pointed to a few disease risks but was remarkably frank in acknowledging widespread uncertainties. Though the authors believed that legalization would expose more people to health hazards, they said the link to lung cancer is “unclear,” and that it is lower than the risk of smoking tobacco.

The very heaviest users can experience symptoms of bronchitis, such as wheezing and coughing, but moderate smoking poses little risk. A 2012 study found that smoking a joint a day for seven years was not associated with adverse effects on pulmonary function. Experts say that marijuana increases the heart rate and the volume of blood pumped by the heart, but that poses a risk mostly to older users who already have cardiac or other health problems.

How Addictive Is Marijuana? Marijuana isn’t addictive in the same sense as heroin, from which withdrawal is an agonizing, physical ordeal. But it can interact with pleasure centers in the brain and can create a strong sense of psychological dependence that addiction experts say can be very difficult to break. Heavy users may find they need to take larger and larger doses to get the effects they want. When they try to stop, some get withdrawal symptoms such as irritability, sleeping difficulties and anxiety that are usually described as relatively mild.