Twenty-nine states have legalized medical marijuana. Among those, nine states allow its recreational use for adults.

As the drug becomes more readily available, some experts have issued new warnings about pot and its increased potency and potential for addiction.

Nine percent of marijuana users will become addicted and that number jumps to 17 percent for those who started using it in their teens, according to the National Institute on Drug Abuse.

A spike in the levels of tetrahydrocannabinol, or THC, found in marijuana today has led to increased concern about the effects on adolescents and their developing brains. It can damage memory and problem-solving abilities, especially in younger users.

THC is the psychoactive part of marijuana that causes the "high."

Last week, Canada became the second country to approve the nationwide legalization of the drug. Uruguay was first. Although The Netherlands openly tolerates recreational marijuana use, it is still technically illegal there.

Is marijuana addictive?

Many pot smokers have long-claimed cannabis is not addictive or that it's less addictive than other drugs, prescribed or illicit, but there has been significant evidence to challenge that claim.

In 2015, about 4 million people in the U.S. fit the diagnostic criteria for a marijuana use disorder, which can take the form of addiction in severe cases, according to the National Institute on Drug Abuse.

A study by the California Society of Addiction Medicine showed that marijuana lights up the brain's reward center by raising levels of dopamine and altering nerve firing. Both are characteristics of drugs that are addictive.

“There should be no controversy about the existence of marijuana addiction,” Dr. David Smith, a physician who has been treating addiction since the 1960s, told the Washington Post. “We see it every day. The controversy should be why it appears to be affecting more people.”

What are the signs of addiction?

The signs of physical and psychological addiction to marijuana are similar to other drugs.

"People who use marijuana frequently often report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness, and/or various forms of physical discomfort that peak within the first week after quitting and last up to 2 weeks. Marijuana dependence occurs when the brain adapts to large amounts of the drug by reducing production of and sensitivity to its own endocannabinoid neurotransmitters," according to the NIDA.

What do you mean it's more potent?

Marijuana growers started engineering cannabis to offer higher THC levels for habitual users who became less sensitive to the effects and as a way to hook new users.

In the early 1990s, THC levels averaged 3.8 percent. That level jumped to 12.2 percent in 2014 and it has climbed from there, according to the NIDA.

This year, the average potency is 20 percent, and that's not the limit.

Extracts known as "dabs," which contain from 40 to 80 percent THC, have grown in popularity, according to a Drug Enforcement Administration report.

Higher THC levels could be increasing the drug's addictive properties, particularly for young people, Smith told the Post.

“Back in the day when kids were sitting around smoking a joint, the THC levels found in marijuana averaged from 2 to 4 percent,” Smith told the Post. “That’s what most parents think is going on today. And that’s why society thinks marijuana is harmless.”

Can you overdose on pot?

While there are no recorded deaths caused by marijuana overdose, a 2016 study pointed to higher THC levels as a possible reason for a 53 percent increase in pot-related emergency transports of teen boys from 2005 to 2010.

Poor performance in school, reduced overall life achievement, greater chances of car accidents, and chronic lung inflammation are linked to teens who regularly use marijuana.