For George, a North Carolina resident now in his mid-60s, quitting marijuana was no problem.

He began using marijuana in college and kept using it after graduation.

“It’s recreational,” he told Healthline in 2016. “Why do people have a drink at the end of the workday? Just because they like it.”

But at the age of 50, he experienced some health problems and decided it was time to quit. For George, that wasn’t a challenge.

“There was no withdrawal,” he said. “There was certainly no physical addiction. If you stopped eating chocolate, you would want to have chocolate again, but it’s not really addictive.”

Millions of other Americans are like George — they can pick up, and put down, marijuana relatively easily.

But that’s not the case for everyone. For some, marijuana poses a substantial risk of addiction.

“I can now admit that I’ve been psychologically addicted to weed for the past decade-plus,” confessed writer Kitty Gray, in a story published in Vice in 2015. “If I need to eat, sleep, relax, be amused, calm down, forget a horrible experience, practice self-love, run errands of any kind, watch TV, or create something: I smoke.”

The National Institute on Drug Abuse reported this month on studies that suggest 30 percent of those who use marijuana may have some degree of “marijuana use disorder.”

They add that people who use marijuana before age 18 are four to seven times more likely to develop this use disorder than adults.

Researchers estimated that 4 million people in the United States met the criteria for marijuana use disorder in 2015. Of them, 138,000 voluntarily sought treatment.

The researchers report the use disorder can morph into an addiction when the person can’t stop using the drug even when it interferes with their daily activities.

The Canyon, a treatment center in Malibu, California, lists 10 signs that you might be addicted to marijuana. Among the signals is a growing tolerance for the drug’s effects as well as using more marijuana than you initially intended to use.

The addiction issue is exacerbated by the fact that 29 states and the District of Columbia currently have laws legalizing marijuana in some form. In fact, nine states and the District of Columbia allow marijuana for recreational use.

In addition, in recent years there have been a number of studies that have extolled the virtues of marijuana.

One stated that medical marijuana can help children with seizures and chemotherapy-induced nausea.

Adult cancer patients are also using marijuana to ease nausea and other symptoms.

People with epilepsy are also using marijuana to ease their seizures.

There are also the studies that indicate legalizing marijuana can reduce the misuse of opioids, alcohol, and other substances.

On the other hand, there are studies that have issued warnings on marijuana and heart health as well as using marijuana for morning sickness.

Plus, pediatricians and others have long said that marijuana can impair brain functions in people under the age of 25.

There’s also the issue of marijuana potency.

In its report this month, the National Institute on Drug Abuse stated that the average THC content in confiscated marijuana samples in 1990 was just under 4 percent. In 2014, it had risen to 12 percent.

This conflict of opinions and research has kept marijuana listed as a schedule I drug by the Drug Enforcement Administration (DEA). Drugs in that category are considered to have “no currently accepted medical use and a high potential for abuse.”

This classification is something that really grates people who support legalization of marijuana.

“It has long been acknowledged that cannabis is a mood-altering substance with some potential for risk, including the risk of dependence. That said, cannabis’ potential risks to health relative to other substances, including legal substances like alcohol, tobacco, and prescription medications, are not so great to warrant its continued criminalization and schedule I prohibited status under federal law,” Paul Armentano, deputy director for the National Organization for the Reform of Marijuana Laws (NORML), told Healthline.

“By any rational assessment, the continued criminalization of cannabis is a disproportionate public policy response to behavior that is, at worst, a public health concern. But it should not be a criminal justice matter.”

Who becomes addicted?

Marijuana use isn’t going away.

A 2013 study concluded that 6,600 people in the United States become marijuana users every day. The number might be higher now because of the states that have legalized cannabis in recent years.

In fact, marijuana use among seniors is even rising.

Given all this, the big question is who gets addicted to marijuana and how does this drug misuse develop.

Genes are one strong predictor of addiction, said Dr. Alex Stalcup, medical director of the New Leaf Treatment Center in Lafayette, California.

Studies of identical twins raised in different families support this theory — they have higher rates of addiction co-occurring (meaning that if one is addicted, the other is at greater risk for addiction) than fraternal twins raised apart.

But family ties may also help some people avoid addiction.

“When we look at the criteria for addiction, it has a lot to do with people tempering their behavior,” explained Carl Hart, PhD, an associate professor of psychology at Columbia University in New York and author of “High Price,” in a 2016 interview with Healthline. “It has a lot to do with responsibility skills.… It’s not perfect, but when you look at the people who are addicted, and you look at people who have jobs and families, they have responsibilities, they’re plugged into their societies, they have a social network, the addiction rates within those kind of groups are dramatically decreased from people who are not plugged in with jobs, families, social networks.”

Those who don’t become addicted also tend to have more options.

“Most of us have a lot of choice in life of things that make us feel good,” said Gantt Galloway, PharmD, executive and research director of the New Leaf Treatment Center and senior scientist at the California Pacific Medical Center Research Institute, in a 2016 interview with Healthline.

“Those who have fewer choices, who perhaps don’t have as rich a set of social interactions because their family life is difficult or because they have emotional problems that are stopping them from forming close friendships… those people may find drugs such as marijuana more attractive and be at greater risk for addiction.”

Another factor that plays a large role in addiction risk is mental illness, which has both genetic and environmental causes.

“Mental health is a huge risk factor for addiction,” said Stalcup. “Drugs work very well, at first, for mentally ill people. If you’re anxious, it’ll go away with a couple of hits, a beer. It’s like magic. But then, the tolerance sets in. So, not only do they need to drink more to relieve the anxiety, but every single time they try to stop, the underlying anxiety comes back worse. We conceptualize it as a biological trap. It works at first, it turns on you, it stops working, and then you still have a problem.”

Stalcup estimates that 50 to 60 percent of the people addicted to marijuana his clinic treats have some sort of underlying mental illness. The majority he sees have depression, anxiety, post-traumatic stress disorder (PTSD), or schizophrenia.

At first, marijuana offers a benefit to each of them. It makes the world more interesting to counteract the loss of pleasure in depression. It soothes anxiety. For those with PTSD who experience nightmares, it shuts down the process by which dreams form in the brain.

The drug of choice

The conversation around marijuana use has become more nuanced since the World War II era film “Reefer Madness” portrayed the drug as destructive and dangerous.

The pain-relieving properties of the drug make it a potential replacement for pain medication. States that had legalized medical marijuana reported in 2014 a 25 percent drop in overdose deaths from pain pills.

In healthy people, marijuana is sometimes used as a substitute for other, stronger substances. Amanda Reiman, PhD, policy manager for the California office of the Drug Policy Alliance, and lecturer at the University of California Berkeley, shed light on this trend.

A study she conducted on medical marijuana users revealed that 40 percent of them had substituted marijuana for alcohol, 26 percent for other illicit drugs, and 66 percent for prescription drugs.

Reasons they gave included marijuana had fewer unwanted side effects, it managed their symptoms better, and it presented fewer problems with withdrawal.

One marijuana user, Conrad, age 47, of San Francisco, said that when he can’t smoke, he drinks more.

“I’ve always found quitting marijuana to be easy when I needed to because of travel reasons or personal reasons, or professional, or what have you,” he told Healthline. “I do know for certain that when I’ve been on vacation for a long time, and obviously I’m not smoking, I subconsciously substitute alcohol. I do drink more alcohol to ‘take the edge off.’”

Building up a tolerance

Dependence on marijuana happens when users build up a tolerance for the substance and need more and more of it to experience the same effect.

When a drug enters the brain, it overrides the brain’s natural processes, boosting a specific function far above, or below, normal levels.

The brain may become resistant to the effects of the drug in an effort to protect itself, so that next time the person uses the drug, it doesn’t have as strong an effect. In order to feel the same high, the person has to take larger and larger doses.

Over time, users may graduate from smoking marijuana to using it in high-dosage edible forms, or propane-extracted concentrates called dabs.

One study found that people who use marijuana have fewer receptors in their brain for endogenous cannabinoids, the signaling molecules that marijuana’s active component, THC, mimics. THC also affects the brain’s reward system and the release of the “pleasure hormone” dopamine.

“It is very well known that dopamine is one of the most important neurotransmitters that regulates reward, motivation, and self-control,” said Dr. Nora Volkow, director of NIDA and one of the authors of the study.

“All of the drugs, whether legal or illegal, that can cause addiction apparently can stimulate dopamine signaling in the main pleasure center of the brain.… By stimulating dopamine, they activate the main reward centers of the brain. This is why when someone takes a drug, it is pleasurable.”

Volkow also conducted a study that found that the brains of people who misuse marijuana have a decreased response to dopamine. When given a chemical, methylphenidate, that caused dopamine levels to rise in the brain, the marijuana users didn’t respond as strongly or feel as high as nonusers. And the more blunted their response to the methylphenidate, the more negative emotions they felt, including irritability, anxiety, depression, and aggressiveness.

“The problem isn’t that they are releasing less dopamine, but that the dopamine stimulation in the brain is having a very attenuated effect,” Volkow said. “The brain doesn’t know what to do with the dopamine. The dopamine signal is not being heard, not communicating properly downstream.”

Volkow thinks that this decreased response to dopamine is likely caused by marijuana use. Another possibility is that marijuana users who become misusers have a dopamine system that’s naturally less responsive, making them more vulnerable to abusing the drug.

“The most common genetic legacy relating to addiction is inherited boredom,” explained Stalcup. “It’s a group of kids we call born bored. What they have scientifically is a pleasure system that’s about 20 percent below normal. When they first try a drug, like cannabis, the lights go on. They say ‘Doc, this is the way I’m supposed to feel. I’m so bored. But I don’t care if I’m bored when I’m high.’

“Here’s the bummer. It does benefit them. Their grades often will go up for a period of time. They’re more sociable. They do more things,” Stalcup added. “The tragedy is, they get tolerant.”

This explanation matches the experience that Gray, the writer for Vice, described.

“I smoke just to get through the boring parts of my day: grunt tasks like making breakfast, showering, running errands, and walking to work,” she wrote.

Her habit had increased from once to at least three times a day, smoking “between one and infinity joints at night, depending on how much weed I have.”

Volkow explained that the patterns of activity in the brain shift from the drug activating reward centers to activating other, nearby regions related to the formation of habits. She said, “They start to recruit instead other [brain] networks that are associated with habits and routines. This allows a transition from a behavior that is predominantly driven initially because it’s pleasurable and rewarding to one that’s automatic because it creates a habit or routine.”

Dependence and withdrawal

Once tolerance sets in, dependence can form. If someone uses a drug often enough, the brain will become accustomed to it.

In an attempt to return to baseline, it will compensate for the difference, raising a function that the drug lowered, like heart rate, or reducing a function that the drug boosted, like mood.

This means that when the drug wears off, the person’s heart could start to race, they could become irritable or depressed, or experience any number of other reactions called withdrawal.

“A person is not dependent on a drug unless they experience some kind of negative outcome upon stopping their use,” said Reiman.

“For example, if I am prescribed Vicodin for pain and I use it as directed, that does not make me dependent. If I try to cut down or stop my intake and have negative consequences — cravings, irritability, upset stomach, chills, etc. — that could be a sign that my use has become dependence. This can happen to people who take prescription medication for a long time, even if they are taking it as directed by their doctor.”

So a drug can cause dependence but not abuse, as is the case for some people prescribed opiate painkillers. Or a drug can cause no withdrawal at all, as in the case of cocaine, but still be quite risky for abuse.

Although not nearly as extreme as heroin or alcohol withdrawal, quitting marijuana does appear to cause withdrawal symptoms in heavy, frequent users.

In a 2013 article for Salon, writer M. Welch described his first week without marijuana after about a decade of daily use as one filled with sleepless nights and irritable days.

“Then, on the fifth day, I began to calm. By the eighth day, the monkey vanished, and I haven’t seen him since,” Welch wrote.

It’s not that easy for everyone, Stalcup said.

“Withdrawal is the mirror image of what the drug does,” he explained. “If cannabis makes you mellow, then you’re irritable, grumpy.”

Instead of marijuana’s sedating effects, a person might get insomnia. Loss of appetite and nausea replace the munchies. And instead of marijuana’s characteristic dream suppression, someone in marijuana withdrawal might have intense, vivid dreams when asleep.

“For many people, that’s really unpleasant,” Stalcup added. “Especially the irritability, that gets a lot of my patients into trouble. I hear, ‘Doc, I open my mouth, all this poison came flooding out of my mouth, I knew I didn’t mean it, I just couldn’t shut up.’”

A pathway to abuse

Most marijuana users never let their use become a problem. They’re not driving high or getting high at work. They don’t get caught with marijuana and never enter the legal system.

Some are even dependent on the drug, using it daily and suffering withdrawal if they try to quit, but still remain functional.

“There are people who have a glass or two of wine a day,” said Hart. “In fact, a glass or two of wine a day is considered healthy.… Now, you certainly might see some sort of withdrawal symptom if someone’s been drinking for a few years and they abruptly stop doing that. But that person, they’re going to work, they’re meeting their obligations, they’re handling their responsibilities. We wouldn’t call that person an addict.”

For some people, however, marijuana use gets out of control and starts to create problems.

“Marijuana-addicted people rarely present for treatment,” said Stalcup. “So a lot of the people we see have gotten caught up in the legal system. The typical example is a 16-year-old who got caught with a bong in his backpack, stoned at school. A lot of our marijuana referrals come through probation, parole, the courts, lawyers, and we see a fair number of those. We see people after they’ve experienced an adverse consequence.”

Hart said, “The bottom line is: ‘Do you have a problem with drugs?’ A problem being defined by having disruptions in your psychosocial functioning. Disruptions in your occupational functioning. Your personal interactions and relationships. Your educational functioning. All these sort of things are disrupted. And that’s what we call substance use disorder.”

One study examined drug users who came to the emergency room with drug-related problems, a strong indicator that something is out of control.

About 90 percent of drug users whose primary drug of choice wasn’t marijuana met the criteria for abuse, compared to 47 percent of primary marijuana users. Of the marijuana users, the 47 percent who met the criteria for abuse were also more likely to smoke tobacco and binge drink than the non-abusers — potential warning signs that those people may naturally run a greater risk of substance abuse in general.

“If you are getting in trouble because you are using or going after an illegal drug, the illegality, and the fact that you don’t stop, and the fact that you keep getting in trouble over it, says that you have a high degree of a substance use disorder, and that you need treatment,” said Michael Kuhar, PhD, a professor of neuropharmacology at Emory University’s School of Medicine, and author of “The Addicted Brain: Why We Abuse Drugs, Alcohol and Nicotine,” in an interview with Healthline.

“If you’re doing something that’s wreaking havoc in your life, you need help. Forget what we call it.”

The cycle of addiction

Unlike opiate misuse, which can set in fairly quickly with heavy use, marijuana misuse can take months or even years to develop. A user might not immediately realize that they’ve crossed the line into addiction.

“A part of the process for some people is to rationalize continued use despite having adverse consequences,” said Galloway. “They may not readily admit to themselves or discuss with others what impact these drugs are having on their life. So, they get stuck in the cycle of use and adverse consequences.”

For many, it’s difficult to imagine a life in which doing drugs is more important than spending time with friends or doing favorite hobbies. It’s certainly difficult to imagine doing drugs despite major consequences, such as a suspended driver’s license or prison time.

But as Galloway explains, an addicted person isn’t making decisions the same way a non-addicted person would.

“Part of the problem with prevention and deciding whether you should use a drug or not is that it’s hard to imagine, with one’s current brain, having a brain that isn’t making those evaluations rationally,” he said.

“You or I, presumably, can have a glass of wine in front of us and decide to pick it up or not. Neither of us feels a lot of compulsion, we feel a lot of choices — we’re going to weigh ‘do I have to drive, do I have work in the morning, am I taking care of a child, how many drinks have I already had?’

“An alcoholic doesn’t weigh things the same way. They look at the immediate benefits and immediate costs to a greater extent than they do at the long-term costs and benefits of using alcohol,” Galloway added. “The person who’s addicted may not think through or may not acknowledge that there are consequences of use — that they’re not going to be as effective at work if they’re stoned, that they’re not going to be engaging with their family as well.”

Stalcup recommends this simple test for addiction. “To make the diagnosis, we propose an experiment. In the experiment, we ask you for a defined period of time not to use. The basic question that we ask is, ‘OK, so you smoke pot, that’s not the issue. Can you not smoke pot?’ Someone who’s not an addict, that’s not a problem. Being unable to not smoke it when you’re trying not to smoke it defines addiction. I encourage anyone who’s using any substance to do this experiment from time to time.”

Editor’s note: This story was originally published on July 20, 2014, and was updated by Rose Rimler on August 9, 2016 and David Mills on May 29, 2018.