Iran has the largest heroin problem in the world. But when Foreign Correspondent Jonathan Miller and filmmaker Mehran Bozorgnia went to meet addicts, they found a progressive treatment programme.

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Warning:the accompanying film contains graphic scenes of drug use which you may find distressing.

Liberal drugs policies are usually associated with liberal countries. So it may be surprising that a strict Muslim state such as Iran not only has the world's worst problem of heroin addiction, but is tackling it with enlightened techniques.

There are 3.5 million heroin addicts in Iran and the widespread sharing of needles is fuelling a soaring rate of Aids infection.

Up to four million Iranians are addicted to opiates - and that could soar to ten million by 2020 if things do not change.

"One-in-three of these people has HIV-Aids and 90 per cent have Hepatitus C."

Iran is a major route for drug smugglers, from neighbouring Afghanistan to the lucrative markets of the west.

But western democracies may have something to learn from the country's programme of methadone treatment.

An Iranian drug addict prepares a dose of heroin before injecting himself. (Getty)

Channel 4 News spoke to drug users crouching in the streets and disused buildings. One addict, 21-year-old Ramin, said: "I have no hope in my life anymore, I've tried to kill myself twice - unfortunately I am still here. No-one likes us in this world."

One-in-three of these people has HIV-Aids and 90 per cent have Hepatitus C.

Needle-sharing is to blame. Perhaps the fear of a looming HIV epidemic is driving the solution.

Jonathan Miller joined up with two men, "Mohammad" and "Mortizah", former addicts, who now work to help others get off heroin.

Every morning they go to the place where they used to shoot up - they are mobbed as they hand out clean syringes and needles. This is all officially sanctioned and subsidised by a government which previousy executed addicts.

Heroin addict "Ibrahim" said: "What these guys have been doing has been really effective - the needle exchange programme has massively brought down the number of HIV and Aids cases around here."

Medical care is also provided for the addicts as well as advice about clean equipment.

The United Nations has praised Iran's "progressive" drugs policy. "Mohammad" now takes methadone, the synthetic heroin substitute. The methadone programme is fully backed by the government - addicts are treated as patients rather than criminals.

"Mohammad" told Channel 4 News: "Methadone is a kind of pill that keeps you away from drugs - and even if the police stop you, they can't lock you up."

"I think methadone is a miracle. I used to inject seven grams of heroin a day. I'd line up five syringes next to each other, stick the needle in my arm and as soon as one was finished, I'd swap the syringe over."

He stole from his mother to pay for his habit. She told us: "He leads a healthy life now. He wakes up in the morning, has a shower and makes his own breakfast - I'm so happy for him."

Doctor Bijan Nasirimanesh is director of the Persepolis treatment centre where Mohammad first got help.

"I think methadone is a miracle. I used to inject seven grams of heroin a day."

He said: "Patients who come here switch from intravenous addiction to methadone treatment and they also get counselling here."

Methadone, a prescription drug, banishes cravings. It is handed out to a queue of patients who must take the pills on the spot to prevent them selling them - or overdosing.

One patient "Yasmin" said: "You feel normal after taking the methadone. It's like you've never taken drugs and you are normal."

Under Ayotollah Khomeini's regime possession of two grams of heroin was a capital offence. Now only traffickers face such punishment.

Channel 4 News captures heroin addicts shooting up in disused buildings.

Mr Nasirimanesh told Channel 4 News: "Contrary to what people think about the current government - that is more conservative and fundamentalist - it supports our programme more than previous governments.

"When President Ahmadinejad was the mayor of Tehran, he ordered the city authorities to build 40 drop-in centres. Now that he has become president he always talks about methadone treatment and is quite open about Iran's drug problem."

Recently the Iranian government made methadone available in jail - a radical measure. In the US not one federal jail offers such a programme. Three-quarters of Iran's prisoners are addicts, and the same proportion has HIV.

Former prisoner "Hussein" explained: "You can score drugs in jail much more easily than outside. When I was inside I used to go the pusher, give him a bit of money and inject with a pump.

"The same pump was used by at least 100 people to inject drugs from morning until night. And it could be used by even more than 100 people - way more."

Iran's methadone revolution means food hand-outs for those who, just a few years ago, would have been hanged. It is a double reprieve from a life which may have killed most of them anyway.