Almost 3,500 children have been treated for substance misuse in London in a year, according to the latest figures.
The breakdown shows that out of the 3,448 under 18s who received treatment, 15 were under the age of 12, 70 were 12-13 and 190 were 13-14.
Three-quarters of these youngsters were treated for cannabis abuse, and just over a fifth for alcohol abuse.
Figures came from a Freedom of Information request to Public Health England, which is responsible for the National Treatment Agency for Substance Misuse.
The number of boys treated for substance abuse in London was 2,328 in 2012/13, and 1,120 for girls in the same year. The overall trend in numbers of young people receiving drug treatment has been falling since 2008-09.
Public Health England said: "Substance misuse among children and young people is always a very serious matter and is seldom the only problem the child is experiencing.
"Often it will be part of a range of difficulties, for example these might include family relationship breakdown, poor school attendance and anti-social behaviour.
"Young children who misuse drugs are likely to be at risk of serious harm and safeguarding needs to be at the centre of a care plan that goes beyond their substance misuse and works with local authority children's services to ensure the child is getting all the support they need."