Obesity, boozing and other lifestyle risks facing teenagers in the rich world are spreading fast to counterparts in poorer countries, a probe published on Wednesday in The Lancet said.
"The high-income world has been grappling with a rising tide of risks for non-communicable diseases, including the problems of obesity, physical inactivity, alcohol, tobacco and illicit drug use," it said.
"That tide is now overwhelming many low and middle-income countries who have yet to bring in measures to control the problems of injury, infectious disease and maternal mortality in this young age group."
The medical journal published a study of health problems facing the world's 1.8 billion adolescents, the biggest tally of this age group in human history.
It defines this term as young people aged 10 to 24, at which the brain is considered to have reached physical maturity.
The risks facing young people are many, but often are poorly understood and undocumented, said the review.
They include deaths from road accidents, which are the biggest single cause of death among adolescents, from suicide or teen motherhood, and illness caused by the AIDS virus and substance abuse.
Globally, South Africa has the highest rates of recorded adolescent mortality, according to the inquiry.
Deaths among adolescent males in South Africa are eight times higher than in rich countries. Among females, the rate is 30 times higher.
Among 27 rich countries, the highest mortality rate was in the United States, mainly due to violence and road accidents, followed by New Zealand and then Portugal.
The safest country in the rich league was Singapore, where the adolescent death rate was just a third that of the United States, followed by the Netherlands and Japan.
The review points to new drivers of ill-health among adolescents, including the marketing of foods that are high in sugar, fat and salt and sales strategies by tobacco firms that aim at teenage girls.
Social media, too, is not without psychological or emotional risks.
The report noted emerging phenomena such as cyber-bullying or "sexting", the act of sending sexually explicit or pornographic messages by mobile phone.