Matthew J Atha on the true level of cannabis usage in Britain; and Josh Kerr on why consumers can’t end the drugs trade

The Institute of Economic Affairs has seriously underestimated the size of the UK cannabis market in their paper on tax revenues in a regulated market (Legalising cannabis could be ‘win-win-win’ for UK, says thinktank, 29 June).

Data from our 30,000 survey respondents shows average annual usage to be 423g rather than the report’s 82.5g, which would put the quantity used by 3 million citizens at 1,270 tonnes rather than the IEA’s 255 tonnes. This would value the UK market at close to £10bn.

This underestimation carries the risk that when post-regulation sales reach our estimates it will be condemned as a big increase in usage rather than recognised as unveiling the true level.

The evidence from abroad suggests increases in post-legalisation usage tend to occur among older age groups, remaining stable or even declining among teenagers and young adults.

Matthew J Atha

Director, Independent Drug Monitoring Unit; author of Taxing The UK Cannabis Market (2011)

• John Keane raises the issue of the morality of using drugs, considering the horrors of the trade (Drug users, take responsibility for the harm that you cause, Letters, 28 June).

I assume he has never owned, for example, an Apple product. And there’s no way an ethical consumer like him drives a car or has more than two children. I would also assume therefore that he doesn’t drink coffee or alcohol or, if he does, he makes sure his choice of product is – in every way – ethically sound.

The point should be obvious, that focusing on the individual consumer is not an effective strategy when faced by oppression and injustice that is systemic – this is a problem with international capitalism generally and not just the drugs trade.

Mr Keane is on the right track but doesn’t cast his net wide enough.

Do I have a solution? Perhaps not, but ethical capitalism is an illusion and Keane should be wary of this line of thinking.

Josh Kerr

Guildford, Surrey

• Join the debate – email guardian.letters@theguardian.com

• Read more Guardian letters – click here to visit gu.com/letters