Assembly figures show pre-teenagers are treated for cannabis and alcohol abuse
More than 100 children aged under 12 were treated for substance abuse in Wales last year.
Most were referred for alcohol and cannabis abuse, but some were also treated for cocaine, heroin and crack addictions.
An education adviser in one of the worse-affected areas said children should be told about the dangers of drug and alcohol as soon as they start nursery school.
The data released by the Assembly government this week relates only to those who turn up for treatment, and it says the figures could be a small proportion of the real number with problems.
Last year, 130 under-12s were treated for substance abuse in Wales, alongside 3,200 children aged 12-19. Cannabis misuse was more widespread among boys than alcohol, with 791 being referred for the former and 770 for drink problems. Teenage girls experienced more problems with alcohol compared with other substances.
But heroin is a growing problem for both sexes, with 118 boys and 114 girls of secondary age referred to drug centres by GPs, youth offending teams or their families. Seventeen of the referrals with heroin addictions were under 12. Amphetamines, ecstasy and solvents are also resulting in young people seeking treatment.
Harry Fisher, a drugs counsellor for young people in Swansea, said his project alone was seeing 20-25 new referrals every three months.
"We have seen more heroin misuse over the past five or six years. It's mainly 16 and 17-year-olds but some are younger.
"The other main substances are alcohol and Valium - mixing them creates big problems."
Mr Fisher said schools must educate pupils about substance abuse, and that it is always better for teachers to do it than for external agencies to give one-off talks.
"Teachers have long-term contact with the children and can develop trust with them," he said.
According to the figures from the Welsh Database for Substance Misuse, the worst-affected area for all age groups in 2005-6 was Swansea, despite a raft of measures by agencies working there.
Mark Campion, the city council's teacher adviser for personal and social education, said schools had an educational role to play from an early age but could not be expected to change youth culture.
"The earlier the age the better so that subliminal messages children get at home about the dangers can be reinforced at school to give them a solid base," he said.
But Mr Campion said that because of finite resources, not enough teachers had PSE training, and support tended to come only at crisis point.
"Most schools do things because it's important to be seen to be doing them, but they rarely evaluate what they are doing.
"We want to make sure drug and alcohol issues are covered as topics at all key stages in an appropriate way. There are plenty of high-quality teaching resources but most people are not trained in PSE."
An Assembly government spokesman said spending on substance misuse had increased from pound;3.3 million in 2003-4 to a draft budget of pound;22m in 2009-10. It is also consulting on a framework for education, prevention and treatment of substance misuse for children and young people.