Addiction treatment centers in Colorado are bracing for an increase in teens referred for marijuana use, ABC News reports. The state began legal sales of recreational marijuana for adults last week.

While only people 21 and older are allowed to purchase marijuana, some experts are concerned the law will allow the drug to more easily fall into the hands of teens.

Dr. Christian Thurstone, who heads the teen rehabilitation center Adolescent STEP: Substance Abuse Treatment Education & Prevention Program, said 95 percent of patient referrals to the program are for marijuana use. In preparation for the new law, Dr. Thurstone has doubled his staff.

He told ABC News that marijuana can be harmful for some teens, particularly those suffering from mental illness. He said that after Colorado legalized medical marijuana in 2009, teens began to use much higher potency products. “Our kids are presenting more severe addictions; it takes them longer to get a clean urine drug screen,” he said. Higher-potency marijuana can increase the risk of psychotic episodes in some teens, Thurstone added.

“Anecdotally, yes, we’re seeing kids in treatment here who have paranoia and seeing things and hearing things that aren’t there,” he said. “Adolescent exposure to marijuana [raises] risk of permanent psychosis in adulthood.”

Ben Court, an addictions expert at the University of Colorado Hospital Center for Dependency, Addiction and Rehabilitation, has also seen an increase in patients addicted to marijuana since the state approved medical marijuana. He says the younger people are when they start consistently using marijuana, the more likely they are to become addicted. “Most people are going to smoke weed and it’s not going to be an issue. By 18 to 24, your odds are less than 1 in 10 that you’re going to be addicted,” he said. “If you start under 18, it’s 1 in 6.”