Just one or two joints is enough to change the structure of a teenager's brain, scientists have warned.
And the drug could cause changes affecting how likely they are to suffer from anxiety or panic, according to a study.
Researchers found 14-year-old girls and boys exposed to THC – the psychoactive chemical in cannabis – had a greater volume of grey matter in their brains.
This means the tissue in certain areas is thicker, and it was found to be in the same areas as the receptors which marijuana affects.
Experts said thickening of brain tissue is the opposite of what usually happens during puberty, when teenagers' brain matter gets thinner and more refined.