City-living significantly increases the risk of psychotic experiences such as hearing voices and paranoia in young people, research has shown.
The study of 2,000 British 18-year-olds found that those growing up in urban areas were 40% more likely to have had episodes of psychosis than their countryside counterparts.
Among teenagers living in the largest and most densely populated cities, more than a third (34%) reported psychotic symptoms between the age of 12 and 18.
Participants were considered to have suffered from psychosis if they had reported at least one of 13 potential experiences, which included hearing voices, believing spies were watching them or their food was being poisoned.
Dr Helen Fisher, one of the researchers from the Institute of Psychiatry at King's College London, said: "These findings highlight the importance of early, preventative strategies for reducing psychosis risk and suggests that adolescents living in threatening neighbourhoods within cities should be made a priority.
"If we intervene early enough, for example by offering psychological therapies and support to help them cope better with stressful experiences, we could reduce young people's risk for developing psychosis and other mental health problems further down the line."
Crime was a strong contributing factor, the study found. Among teenagers who had grown up in the most deprived neighbourhoods and been a victim of violent crime, 62% reported psychotic experiences.
Adolescents are especially vulnerable to developing mental health problems and around 70% of adults with psychiatric illness are thought to have become unwell during their early teenage years, said the researchers whose findings are reported in the Schizophrenia Bulletin journal.
Up to one in three young people are thought to encounter psychotic experiences at some point. These individuals are at an increased risk of schizophrenia and other forms of mental illness as adults.
They are also more likely to take their own lives than other members of the population.
Previous research by the same team found higher rates of psychotic symptoms among children living in cities.
Co-author Jo Newbury, also from the Institute of Psychiatry, said: "Our study suggests that the effects of city life on psychotic experiences are not limited to childhood but continue into late adolescence, which is one of the peak ages at which clinical psychotic disorders are typically diagnosed."
Brian Dow, from the charity Rethink Mental Illness, said: "With young people at such an increased risk of developing psychosis when exposed to the stresses and strains of city living, ensuring someone can get quick access to support, such as psychological therapies, can make all the difference in their recovery.
"There is a melting pot of factors that can contribute to someone experiencing a condition such as psychosis, from genetics, environmental and biological factors, and symptoms can be complex or hard to make sense of. If you are worried about your mental health, talk to your GP or contact our advice service for more information."