Crack cocaine, along with heroin, is one of the most addictive drugs
One in three people addicted to heroin or crack cocaine in community drug treatment programmes in England stops using by six months, research suggests.
Results from 14,600 people in methadone or specialist counselling programmes suggested the rates were slightly lower for those dependent on both drugs.
There are about 140,000 people in such treatment programmes in England.
The study, in the Lancet, showed higher funding for treatment was effective, the Department of Health said.
The researchers said the figures were "encouraging".
Eight in every 1,000 15-64 year-olds are heroin users, statistics show.
For crack cocaine - the smokeable form of the drug - the figure is five in 1,000.
The latest analysis is the largest study done in England of 1,000 community treatment agencies - not including residential rehab schemes - and may be the largest internationally, the researchers said.
Data was collected as part of the National Drug Treatment Monitoring System, where 13,200 people had started on drug treatment such as methadone, and 1,400 people started on psychological treatments - the only current option for crack cocaine addiction.
Among those addicted to heroin, 37% said they were abstinent from the street drug for the month prior to their six-month review.
Another third were classed as "improving" because they had reduced their drug use, just under a third were unchanged and 3% had got worse.
For crack cocaine, 52% had stopped using at six months, 12% had improved, a third were unchanged and 3% had deteriorated.
The number of people addicted to heroin and crack in treatment programmes in England has increased dramatically in recent years.
It has been estimated that in England around 60% of those in need of treatment are receiving it, at a cost of around £3,000 to £5,000 a year per person.
Colin Bradbury, head of delivery at the National Treatment Agency for Substance Misuse and one of the authors of the study, said: "There's no room for complacency, but we're very pleased and encouraged by the results.
"That is despite the fact we have had very rapid expansion in the treatment system, it's retained its quality and effectiveness."
He added there would be large local variation and services could use the study to benchmark how they were doing.
Study leader Dr John Marsden, from the Institute of Psychiatry at King's College London, said these were the most "severely addictive" drugs around and users could not stop even when their lives fell apart and it became completely illogical for them to use.
"These are encouraging rates but it is the beginning of a story."
He said they would be doing more work on long-term outcomes and different types of treatment.
"I think this is good evidence for a return of public investment in treatment."
He called for more attention on those who got worse during their treatment programmes and new strategies for the third of users addicted to both heroin and crack cocaine who seemed more resistant to treatment.
A Department of Health spokesperson said: "This study demonstrates that our strategy of increasing investment in treatment is effective and gives problem drug users faster access to help.
"Tackling drug misuse is a priority. High-quality drug treatment is the most effective way of reducing drug harms and every £1 spent on drug treatment saves £9.50 to the rest of society."
In an accompanying paper, Dr A Thomas McLellan, deputy director of the White House Office of National Drug Control Policy in Washington DC, said short-term interventions may not produce long-lasting, positive effects.
"It may be more reasonable to expect enduring improvements through sustained outpatient clinical management with drugs and behavioural therapies - like expectations we have for the management of diabetes and hypertension."
A spokesman for Addaction said the study highlighted the problems of "polydrug use".
"Many of the people Addaction helps use a wide range of substances, and treating their addictions is a complex business."
DrugScope chief executive Martin Barnes said drug treatment made a difference to thousands of people.
But he added: "Despite the improvements that can be made it is important to recognise that getting off drugs can be a long and complex process - there is no silver bullet for treating drug dependency."