Lighting it up before school might not be as cool as you think. A new study, published in the New England Journal of Medicine, links smoking marijuana in kids with lower intelligence and poorer focus, and may translate into long-term effects.

A recent National Institute on Drug Abuse (NIDA) report says that in 2013, an estimated 22.7 percent of high school seniors were smoking weed, with 6.7 percent of them smoked it dail. Those numbers are rising, too. The idea that marijuana is a harmless substance to kids is very concerning for researchers, who found that regular use of the drug over time, especially among younger kids, can impair brain development during a critical time in their lives.

"Long term, heavy use of marijuana results in impairments in memory and attention that persist and worsen with increasing years of regular use,” the report said, according to Time. With two states, Washington and Colorado, legalizing recreational use of weed, the drug has the potential to be even more harmful to children. Some argue that easier access will reduce smoking risks, but with drug use among children rising, researchers are trying to understand the impact on the mind. The new research also reveals that kids who smoke pot might be more likely to dropout, to participate in criminal activity, and to have lower grades.

The scientists reported that adults who started smoking as adolescents had “impaired neural connectivity," which affects memory, alertness, and processing of basic routines. In turn, the ability to learn is worsened. Researchers also found that impaired brain functioning occurred for a few days after smoking weed, affecting a child’s ability to perform at their greatest potential in the classroom.

According to NIDA, other side effects of marijuana smoking include rapid heartbeat — which also causes the eyes to look red. This usually happens only a few minutes after lighting up. The drug also slows reaction times, which can result issues with response to signals and sounds. Their research also correlates smoking weed over time with an increased risk of developing psychosis, a mental disorder that causes you to lose sight of reality. People with psychosis have delusions, hallucinate, and hear things that aren’t really there. Smoking weed is also associated with emotional disorders, such as depression and anxiety.

Researchers still have a long way to go to fully understand all the negative effects of marijuana on a child’s brain, but what they have found may help explain why some kids have trouble learning and excelling to their highest capacity.

Source: Volkow N, Baler R, Compton W, Weiss S. Adverse Health Effects of Marjuana Use. The New England Journal of Medicine. 2014.