The NHS attracts only occasional media attention in the US, but the recent debate about top-up funding and co-payments for high-cost cancer drugs was covered by The New York Times.

One article focused on NHS patients who were paying privately for Avastin, but there was no mention that two years earlier the same newspaper had reported on the financial problems many Americans experienced in accessing the drug because the co-payments required were around£5,000-£10,000. The controversy in the NHS must seem strange to Americans, as co-payment for medical care is the norm in the US, even among those with extensive insurance policies.

Many people in the UK are opposed to any form of co-payment because it is seen as counter to one of the core principles of the NHS - that healthcare should be provided on the basis of need rather than the ability to pay.

The theory is that co-payments reduce so-called "excessive demand" for healthcare. There is strong research evidence from around the world that they do reduce usage rates, although whether it is only "excessive" or also "legitimate" demand is much less clear. In addition, the reduction is nearly always disproportionately large among less wealthy and ethnic minority groups.

In the US the impact of top-up payments is clear. A survey from the Commonwealth Fund think tank published in November 2007 reported that 37 per cent of adults in the US had a financial reason for not accessing healthcare in the past 12 months, compared with 8 per cent in the UK. In February there were two surveys about prescription medicines on opposite sides of the pond. The US survey reported that 29 per cent of Americans had not filled a prescription in the last two years because of cost, while 23 per cent stated they had taken a lower dosage than prescribed to make the medication last longer. In the English survey, 2 per cent of people said they had failed to collect a prescription because of charges.

Top-up payments have other effects besides reducing demand. Many in the US believe that by partially paying at the point of use they are able to exercise choice, not just over their doctor and hospital but also over their treatment. Although many Americans are not in the position to make these choices because they have insufficient or no insurance, and the reality among those that do have coverage is that their contribution is often minimal (my co-payment is£5, which is 2.5 per cent of the cost of a routine primary care doctor's appointment), the psychological effect of writing a cheque directly to the physician is significant.

Co-payment turns patients into consumers. Patients have needs that require healthcare interventions and so they seek the help and advice of a professional. Consumers demand a service and thus want to choose the provider who can best meet this demand and give them satisfaction. Though co-payments may assist in encouraging patients to take an active role in their healthcare, they can also result in changing the nature of the medical consultation.