The world spends astonishing amounts of money each year on expensive drugs for things cheaper versions do equally well. If we could find a way to stop this, the savings could make many currently unaffordable treatments available. In an effort to understand what is causing the problem, Oxford University's Dr Ben Goldacre looked at a distinctive aspect of the UK healthcare system, which he calls dispensing practices.

In-house dispensaries, particularly in rural areas, allow patients to collect their drugs more quickly and conveniently, rather than having to go to a separate location. Sadly, there is a catch. Pharmacies, whether in-house or not, can make profits on drugs by negotiating discounts with suppliers, while being reimbursed by the NHS at full price.

The more expensive the drug, the larger the potential profit, which in theory gives doctors at these practices an incentive to prescribe more expensive drugs when cheaper ones will do just as well. Goldacre set out to determine if this means doctors working at dispensary practices prescribe expensive drugs more often than their counterparts without the incentive.

In BMJOpen Goldacre reports this is indeed the case. Goldacre and his co-authors looked at four classes of drugs with large price ranges, and no evidence the expensive versions outperform the cheaper ones, except in rare circumstances.

In all four cases, dispensary practices chose the high-cost option more often than their non-dispensary counterparts, although the extent varied dramatically. For proton pump inhibitors, used to reduce stomach acid, there was only a small difference by types of practice. On the other hand, the chances of walking out with a high-cost angiotensin receptor blocker, rather than a cheap version of these anti-heart failure drugs, was more than five times higher in a dispensary practice than from another GP.

Goldacre also reports the discrepancy was greater for practices that treat larger numbers of patients.

The cost is a direct transfer of millions of pounds from British taxpayers to large pharmacology companies, and to the clinics.

Cynics may respond to these conclusions were so inevitable they were not worth testing, but the work confirms something many doctors don't like to admit – financial incentives sway their prescribing practices.

It's well-known pharmaceutical companies go to great efforts to get doctors to choose the more expensive options, from providing them with pricey gifts to using astonishingly good looking sales reps to argue their case.

However, solid evidence of doctors being swayed by direct financial incentives is rarer. The example Goldacre found may be distinctive to the UK system, but almost certainly has parallels elsewhere in the world.

Solving it, however, may not be so easy, as Goldacre himself acknowledged.