A new report has found that teenagers are at risk of permanently damaging their brains by smoking marijuana.
Statistics show that by the age of 16, one in three kids have tried the drug, even though it's against the law.
Swedish scientists found that teens who regularly use the drug are at most risk from mental illness, because their brains aren't fully formed yet.
The government is thinking about tightening up the laws surrounding the use and sale of cannabis.
People once thought the drug was relatively safe, and although there's no hard evidence it causes mental illness, more and more scientists think it plays a big part.
Problems they have linked to the drug include sleeplessness, fear and other, more serious things like psychosis and schizophrenia.