Repeat something often enough and it becomes a truth. Repeat it a few more times and it becomes a Twitter hashtag. At least that’s what’s happened to the most widely cited figure for the cost to UK society of alcohol harm.

On September 5, at 1.30pm, #21billion will be scatter-bombed across the social media network in a Thunderclap event organised by the Alcohol Health Alliance. You have been warned.

It’s not just that it’ll clog up your Twitter feed. The question is, how did the £21 billion become almost universally accepted as fact?

In a recent blog post beer writer Pete Brown was blunt about it. “When will the anti-alcohol lobby stop lying?” he asked.

His frustration is understandable. A couple of years ago the source of the £21 billion figure was investigated by independent fact-checking body Full Fact. It traced it back to a document published by the Cabinet Office in September 2003 (now available on the Alcohol Research UK website).

Its upper cost estimate, at 2001 prices, is a shade over £20 billion. In the intervening years various bodies have made their own adjustments to the figure, but somewhere along the way it’s been rounded up. Perhaps because £20 billion sounds too much like a rounded number.

More to the point, Full Fact had a number of questions about how the total was arrived at, including what some of the components actually mean. Unable to find anyone from the Cabinet Office to answer those questions, it did “not feel able to rely upon the £21 billion cost figure”.

Others, inside and outside public health circles, have not felt such qualms. It’s prominently cited in the current government Alcohol Strategy document, in the manifesto recently released by the All-Party Parliamentary Group on Alcohol Misuse and at every available opportunity by the mainstream media.

Surely so many can’t be wrong? Well, perhaps they can.

In August 2012 the international public health community suffered an Emperor’s New Clothes moment. One of their number, Professor Klaus Mäkelä, writing in the academic journal Nordic Studies on Alcohol & Drugs, declared that attempts to aggregate the social costs of alcohol harm are worthless.

“Even the most sophisticated cost-of-alcohol calculations include entries based on misleading assumptions or logical mistakes,” he said.

For example, some £4.5 billion of the £21 billion (or rather £20 billion) is attributed to lost output due to reduced employment and premature death. Unless the UK is a utopia of full employment this is a nonsense. In reality anyone who is unemployed, or who dies, because of their drinking does not leave an unfilled hole in the labour force – they are replaced.

Adding in the cost of property stolen in alcohol-related crime is another error, Makela argues. The Cabinet Office asserts that because stolen goods are removed from the legal economy they are a loss to “society as a whole”, but that cannot be true. The black economy is still part of society and it interacts, too, with the official economy.

Then there are alcohol-related hospital admissions. The Cabinet Office helpfully separates costs deemed directly and partly attributable to alcohol, coming up with £126 million for the former and £400 million for the latter.

Late last year, however, Public Health England announced it was going to stop using partly attributable admissions in its figures, which are based not on an actual count of patients with a drink problem but on adding up “alcohol attributable fractions” based on the likelihood that drink may have been a factor in an illness.

This sensible decision reduces the total hospital admissions from 1.2 million to about 300,000 – as well as slicing £400 million off the social cost of alcohol on Cabinet Office figures.

The costliest single element in the calculation, however, is for the emotional impact of alcohol- related crime, which comes in at £4.7 million. This seems to be made up of a mixture of health costs and lost production caused by taking time off work, but it isn’t entirely clear how that particular number was arrived at.

And if anyone is bothered about bringing these 13-year-old stats up to date they might note that alcohol-related crime has fallen by at least 25%.

There are real costs associated with alcohol harm, to individuals who fall ill or lose their jobs, and their families. But there’s something wrong in trying to stick a price-tag on all that, and in trying to generate as large a figure as possible for the politicians and headline-writers without worrying too much about accuracy.

In fact, the debate in the highest echelons of the alcohol research and public health community prompted by the Mäkelä paper suggests the business of producing a number for the social cost of alcohol is a moral and political matter, rather than one of evidence-based science.

Writing in the following issue of the same Nordic alcohol journal, Professor Thomas Babor, lead author of the bible of medical temperance, Alcohol: No Ordinary Commodity, declares that cost-of-alcohol calculations “serve other functions” than science.

“In a democracy, politicians and policymakers often need to be shamed into doing the right thing, and costs to society have the ability to shame, blame and even defame … It is the simple, single monetary figure that captures public attention more than anything else.”

So there you have it. This is not about understanding a problem but “doing the right thing”. Who decides what the “right thing” is, of course, is a big question.

But we can begin to answer Pete Brown’s little question about why the anti-alcohol lobby keeps using the discredited £21 billion.