Around one in five US high school students use cannabis, and some use it daily, yet experts say even once-a-month cannabis use can still cause problems. However, the new warning is that “occasional” does not always mean harmless.

A US study found that using cannabis just once or twice per month links to emotional distress in teenagers and lower school performance. The researchers analyzed a nationally representative survey of more than 160,000 US students, and over a quarter reported cannabis use.

Less than 20 percent said they used cannabis monthly or less often, while a smaller share reported weekly use or almost every day. Meanwhile, scientists worry because today’s cannabis products often contain two to three times more THC than in the past.

Dr Ryan Sultán, who led the study in the journal Pediatrics, said any cannabis use may put kids at risk of falling behind in school, with the most frequent users facing the greatest risk. He added that even a few “harmless” joints can lead to real academic consequences, and that regular users may struggle to focus, miss school, and lose interest in future plans.

Dr Sultán said it is not uncommon for a teen to smoke marijuana only a few times before showing withdrawal signs and a worse mood, even as many other substances sit at record-low use among US teens. Therefore, Dr Tim Becker said the teen brain is still building circuits for learning, self-control, and emotional regulation, and cannabis use during these periods can disrupt development, so experts urge early, frequent parent talks and watching for falling grades, mood changes, or lost hobbies, while FRANK offers confidential drug advice by calling 0300 123 6600, texting 82111, using their website 24/7, or livechat from 2pm–6pm daily.