In the ’60s, marijuana was a hallmark of the counterculture, along with free love, bell bottoms, long hair and bandannas. But marijuana has had the most staying power.

This month, in a remarkable first, the recreational use of marijuana became legal (depending on your definition) in Colorado and Washington. Over a dozen other states have decriminalized possession of small amounts, and Massachusetts recently became the 18th state to allow its use for medicinal purposes.

Though federal law still bans both the sale and possession of marijuana, President Obama has said the federal government has "bigger fish to fry" and won’t aggressively prosecute tokers in states where its use is legal.

The rise of marijuana as an adult pastime is a victory for those who’ve always felt that its hazards were overblown. Proponents of legalization argue that marijuana is much safer to use than alcohol, pointing out that it is virtually impossible to overdose on marijuana.

While marijuana can be addictive, scientists generally agree that fewer than 10 percent of marijuana smokers become dependent on the drug, compared with 15 percent for alcohol, 23 percent for heroin and 32 percent for tobacco. Marijuana does contain carcinogens, including tar and other toxins similar to those found in tobacco, but people generally do not smoke marijuana in the same amounts as cigarettes.

Still, legalization takes health consumers into murky territory. Even though marijuana is the most commonly used illegal drug in the United States, questions about its health effects remain.

For starters, this is not your parents’ pot. Today’s marijuana is much more potent: The mean concentration of THC, the psychoactive ingredient, in confiscated cannabis more than doubled between 1993 and 2008.

Increased potency may be having unforeseen consequences. The human brain’s cannabinoid receptors are typically activated by naturally occurring chemicals in the body called endocannabinoids, which are similar to THC. There is a high density of cannabinoid receptors in parts of the brain that affect pleasure, memory and concentration. Some research suggests that these areas continue to be affected by marijuana use even after the "high" dissipates.

"It’s much more potent marijuana, which may explain why we’ve seen a pretty dramatic increase in admission to emergency rooms and treatment programs for marijuana," said Dr. Nora D. Volkow, director of the National Institute on Drug Abuse. "When we hear, ‘Well, I smoked and nothing happened to me,’ we need to think about the context of when these people started to take it, how frequently they used and how active the marijuana was."

Those in favor of legalizing marijuana say the increase in potency has been exaggerated, and that when users have more powerful pot, they adjust their consumption and actually smoke less.

Teenagers may be more vulnerable to addiction, however, and those who start smoking pot at a younger age are at higher risk. Approximately one in six will become addicted, Dr. Volkow said. Young adults who start smoking marijuana at earlier ages also tend to smoke much more, and more often, than those who start in their later teens, researchers say.

In users who develop a dependence or addiction, quitting can cause intense withdrawal symptoms, like anxiety, trouble sleeping, lack of appetite, mood swings, irritability and depression, experts say.

Both Colorado and Washington restricted marijuana use to adults age 21 and over when they legalized recreational use in November. But experts worry that the perception of marijuana is changing because its stigma as an outlawed drug has eroded.

"When people can go to a ‘clinic’ or ‘cafe’ and buy pot, that creates the perception that it’s safe," said Dr. A. Eden Evins, director of the Center for Addiction Medicine at Massachusetts General Hospital in Boston. "Before we unleash the powers of the marketplace to woo people to use this addictive substance, we need to better understand who is at risk."

"Once moneyed interests are involved, this trend will be difficult to reverse," she added.

The most disturbing new studies about early teenage use of marijuana showed that young adults who started smoking pot regularly before they were 16 performed significantly worse on cognitive tests of brain function than those who had started smoking later in adolescence. They performed particularly poorly on tests assessing executive function, which is responsible for planning and abstract thinking, as well as understanding rules and inhibiting inappropriate responses.

Imaging scans also found detectable differences in how their brains worked, said Staci Gruber, the lead author of these studies and director of the cognitive and clinical neuroimaging core at the imaging center at McLean Hospital in Boston. Imaging scans found alterations in the frontal cortex white matter tracts of the brain in the early-starters, she said, that are associated with impulsiveness.

"The frontal cortex is the last part of the brain to come online, and the most important," Dr. Gruber said. "Early exposure perhaps changes the trajectory of brain development, such that ability to perform complex executive function tasks is compromised."

A recent study showing a drop in IQ scores among teenagers who are regular pot smokers is especially troubling, Dr. Evins said. A more recent study found that people who started smoking marijuana as teenagers and used it heavily for decades lost IQ points over time, while those who started smoking as adults did not, though some critics have said these differences may not be meaningful. Older survey studies had indicated that regular pot smokers were less likely to graduate from high school or pursue higher education, but it was never clear which came first, difficulty in school or the drug use.

"If parents who are spending thousands of dollars on SAT prep courses knew about the cognitive effects marijuana has on their kids’ brains, they would be up in arms," Dr. Evins said. Other health concerns about marijuana are less well documented but may turn out to be significant. States that legalized marijuana prohibit driving under its influence, and studies have found marijuana smoking increases weaving between lanes and slows reaction times. And although marijuana is not as damaging to the lungs as tobacco, in part because people do not smoke a pack of joints a day, a regular habit can eventually take a toll on the lungs.

At the very least, the new studies suggest parents who recall their own pot parties may want to suggest greater moderation to their children. And teenagers who insist on trying marijuana are better off waiting until they’re older.

"It’s the same message as with alcohol," Dr. Gruber said. "Just hold on, it’s worth it to wait."