How did you get involved in The House I Live In [Eugene Jarecki's prize-winning documentary on US drugs policy]?

He just called and asked if I'd talk about the drug war. At that time I didn't have much expectation. I'd done interviews before about the drug war and they had not come out with any complexity in thought. It's my own fault – I hadn't researched Eugene's work well enough when I first encountered him. It was only subsequent to the interview that I saw Why We Fight and realised he was for real.

The film puts you in the position of being spokesperson against the "war on drugs". Is that a role you're happy to play?

I've been doing it now since I worked on The Corner [forerunner of Simon's influential HBO TV series, The Wire]. And I was working on that book in '93. That was the journalism that convinced me the drug war was untenable. If you read The Corner, it's pretty clear that I have little faith in the idea of a drug prohibition. But The Wire was an opportunity to be more editorial about it. The Wire is a very fundamental argument against the drug war, and since it came out I've been speaking very bluntly about the low regard I have for drug prohibition.

The picture painted in The Wire and The House I Live In is bleak, and people don't like to look at bleak pictures. How can America turn and face this huge problem?

It won't happen from leadership. There are two things politicians in my country pay attention to. One is money and the other is votes, and the two are inextricably linked in many respects. For a long time the inner city hasn't voted. In the inner city you have an incredibly disenfranchised American population that understands the burden of the drug war. One of the fundamental ways in which they've disconnected is that if you're convicted of a felony you lose your right to vote for ever. So this is an agenda that has no immediate gain for a politician. That's why jury nullification and a refusal to co-operate with drug prohibition is going to be a grass roots movement.

Are you for decriminalisation or legalisation?

I'm for decriminalisation but I don't care. I don't want anybody in jail for using drugs. I think that's a waste of our money. A waste of resources that could be used elsewhere. I think it's treating a symptom. Drugs are a destructive force in human lives and I've seen what they can do, but the war against them has wiped out whole communities. If it was draconian and it worked, that would be one thing. But the purity level of drugs after half a century of this crap is higher than ever. They haven't taken back a single drug corner in my city [Baltimore]. All it's done has burden us with the highest level of incarceration in the world. We have more people in jail than China. Not higher per capita but in total.

This question of decriminalisation and legalisation does seem critical…

No, it's not. It only seems to be critical to people on the outside of what's happening. Let me put it this way: drugs are legal in Baltimore. Right now there are 60,000 addicts in Baltimore. If they could lock up 100 or 200 of them a day, they would have a record number of arrests. But they can't, that's too many. By the numbers, not by the fucking law, drugs are legal in my city.

Decriminalisation still leaves an international criminal network and distribution business in place. With legalisation there is a basis on which to start unpicking all of that.

When I make the distinction for decriminalisation I don't care about laws any more because the first step will not be to change any laws. And certainly there will not be a sufficient number of politicians with enough courage to legalise drug use. The mistake you're making is that you're leading from the rear. You're having a dilettante's argument about something that will never be considered by the political infrastructure. Getting them to stop jailing people for this crap is plausible. To start talking about legalising heroin and cocaine, you might as well go to a university and shave your head into a point.

Marijuana has been legalised in Colorado and Washington. Do you see any sign of a larger movement?

While I see the changes in marijuana laws as good, I do worry that if they could fix it that white suburban people could get high in their own way, it would be a return to apathy about what's happening in the rest of the country. I don't mean to sound like a complete cynic but I am when it comes to the drug war. This is not a war to control dangerous drugs. This is a war to control the poor.

You've said of the American system that you don't think things will get better.

Unless you marry capitalism to a social compact that has meaning, all it is is a tool for generation of wealth but not for building a functioning society. I read the eulogies for Margaret Thatcher and I thought that woman mistook capitalism as the framework by which you build a just and viable society. When someone starts saying there is no such thing as society, there's only you and your family, that's a horse with blinkers on.

You take a keen interest in the newspaper industry. What do you think about proposals to regulate the press in Britain.

There should be no prior restraint. You already have too much prior restraint of the British press. I couldn't operate under your press law, couldn't do good journalism consistently. Your ability to criticise people in public or reveal secrets that are in the public's interest are much more constrained than ours. And I find that to be unworkable in terms of democracy.

You're working on several projects, one on the CIA. Will they materialise?

No, there's no green light. I'm working on scripts. The company's called Blown Deadline Productions for a reason. We don't turn in scripts until we're totally satisfied with the story. When I ask HBO for tens of millions of dollars, I've got to believe we're making the best possible story or I'm an asshole. I feel that way about everything and therefore generally I'm late on everything.

Join David Simon to debate the war on drugs with film director Eugene Jarecki and others at the Royal Institution, London, on Thursday 23 May. The event will be hosted by Observer editor John Mulholland. Buy tickets at: guardian.co.uk/extra