Teenagers who smoke and drink alcohol are causing visible damage to their arteries by the age of 17, a study has revealed.
Tests showed stiffening of the arteries had begun by this relatively young age.
These physical changes have been linked with an increased risk of heart and blood vessel problems, such as stroke and heart attack, in later life.
But the study also found that the arteries of teenagers who stopped smoking or drinking returned to normal.
The researchers studied data between 2004 and 2008 from 1,266 youngsters taking part in the Avon Longitudinal Study of Parents and Children (ALSPAC), which charts the health of 14,500 families in the Bristol area.
The participants detailed their smoking and drinking habits at the ages of 13, 15 and 17 and tests were then done to discover if there had been any stiffening of their arteries.
They had to report facts including:
How many cigarettes they had smoked in their lifetime
The age at which they started to drink alcohol
Those who had smoked more than 100 cigarettes or tended to binge drink had a higher incidence of stiffened arteries than those who had smoked fewer than 20 cigarettes or had fewer than two drinks per day.
The report's senior author, Prof John Deanfield, from the UCL Institute of Cardiovascular Science, said: "We found that in this large contemporary British cohort, drinking and smoking in adolescence, even at lower levels compared to those reported in adult studies, is associated with arterial stiffening and atherosclerosis progression.
"However, we also found that if teenagers stopped smoking and drinking during adolescence, their arteries returned to normal - suggesting that there are opportunities to preserve arterial health from a young age."
Dr. Marietta Charakida, who carried out the research at the UCL Institute of Cardiovascular Science, said: "Injury to the blood vessels occurs very early in life as a result of smoking and drinking and the two together are even more damaging.
"Although studies have shown teenagers are smoking less in recent years, our findings indicated approximately one in five teenagers were smoking by the age of 17.
"In families where parents were smokers, teenagers were more likely to smoke."
Prof Metin Avkiran, associate medical director at the British Heart Foundation, which part-funded the research, told the BBC that the fact damage could be reversed was an "encouraging indication".
He said: "It's never too late to make changes that may literally end up saving your life.
"This study suggests that the damage to arteries can occur even in the young, leading to serious trouble later on in life.
"Quitting smoking is the single best thing you can do to protect your heart.
"If you do drink, try to ensure that it's not to excess and within the recommended guidelines."