Teenagers in Swansea say they are smoking heroin because cannabis is almost impossible to buy.
A tenth of a gram costs just £10, say users in Swansea
The Class A drug can be bought for just £10 for a small bag of about 100mg, BBC Wales' Dragon's Eye was told.
A homelessness charity in the city said youths mistakenly believe they could use the drug safely.
Police confirmed that heroin's street price had halved in recent years but said Swansea had less of a drugs problem than other areas.
One youth told the programme it was very easy to buy heroin there.
He said: "You either know a dealer or you know someone who knows a dealer... some drugs are drying out and people are turning to harder things."
He said the price of heroin on the streets was £10 for 0.1 grams.
Another teenager said: "I've lived in Birmingham, Blackpool, Stoke-on-Trent, Leicester. The worst place for drugs is Swansea."
Sarah O'Leary, of the Swansea Young Single Homeless Project, said the youngsters often believed they could handle the hard drug.
She said: "They do start using once or twice a week then in a couple of months its four, five, six times a week...(in) a year's time they could be using it every day.
"They just smoke it, then, when they don't get the same effect off smoking it they'll start injecting it and that is when it becomes a real issue."
Plaid Cymru AM Leanne Wood, who used to work as a probation officer, said she had seen numerous people who wrongly believed they could keep their heroin use under control.
She said the only way homeless teenagers could fund that sort of habit was by turning to crime.
A South Wales Police spokesman said the force was using "all available enforcement methods to arrest and deal with offenders."
He said: "We also work closely with local treatment agencies and wherever possible offer the facilities that are available to users of the drug in an attempt to break their habit."
The spokesman wanted to dispel any impression that Swansea's drug problem was particularly acute.
He said: "I can reassure you that this city has a much less problem with drugs than other places in England and Wales."
Dragon's Eye is on BBC 2W at 2100 GMT on Thursday, and on BBC One Wales at 2235 GMT.