Half of young people using cannabis suffer side effects such as paranoia and blackouts, a UK survey suggests.
The effects of the drug on the developing brain are unclear
More than 80% of the 727 young people in their teens and early 20s polled by YoungMinds had tried the drug - the vast majority before they were 18.
The charity is calling for urgent research on the effects of cannabis on the developing teenage brain.
It is releasing guidance for young people and professionals on the effects cannabis may have on mental health.
Barbara Herts, YoungMinds chief executive, said: "Many young people are experimenting with cannabis from a young age.
"We are extremely concerned that there is still very little known about the effects of cannabis on the developing teenage brain and it is crucial that more studies are carried out in this area."
She said virtually all of the research on both short and longer-term cognitive effects has been conducted on adults.
This is a problem as the young, developing brain could be much more vulnerable to its effects, she explained.
Ms Herts said studies show young people who use cannabis regularly or heavily are at least twice as likely to develop a psychotic mental disorder by young adulthood than those who do not smoke.
Psychosis is a type of mental health problem, which includes conditions like schizophrenia, that can seriously affect the way you think, feel and behave.
She said: "Young people, their parents and the professionals working with them need to understand the issues surrounding cannabis use and the potential dangers to their mental health and wellbeing."
Drug helpline Frank recently launched a TV ad warning young people of the damage cannabis can do to the brain as part of its drugs awareness campaign.
A spokesman said they were particularly targeting 11- to 17-year-olds.
Martin Barnes, chief executive of DrugScope, said: "Although cannabis use has been falling among young people over recent years, and is now at its lowest level for nearly a decade, we cannot be complacent.
"There is evidence that cannabis not only worsens existing mental health problems but may trigger - although the risk is thought to be small - some conditions such as schizophrenia.
"We support calls for more research into the possible long-term effects particularly of drug use among children and young people."