It's unusual for me, but today I am actually feeling sorry for a drug company. As a GP, I'm not known to. I have signed up to No Free Lunch - an organisation which believes pharmaceutical promotion should not change clinical practice - and so I don't take freebies from pharmaceuticals and I don't see drug reps. But a small drug company, Denfleet, it was reported at the weekend, has made a cheaper version of the schizophrenia drug clozapine.

Until recently the only version of clozapine was made by Novartis - Clozaril. The new version, Denzapine, may be cheaper but it hasn't been universally welcomed. The Office of Fair Trading is investigating claims that representatives from Novartis have been "talking down" Denfleet's version of the drug and breaching the Competitions Act.

Because clozapine has rare but serious side effects, it has to be carefully monitored and this is one of the reasons why, it is claimed, the new version of the drug might not be as good - there are thorough monitoring systems in place for the Novartis version. But it is tempting to think that Novartis is also bothered about a potential loss of profit: Michael Marven, the chief pharmacist at the Oxfordshire Mental Health Trust, has estimated that a switch to Denfleet's cheaper version would save the trust £170,000 a year.

In many ways, there is nothing much new about the competition between branded products versus generic versions. For years doctors have been encouraged to prescribe them. While the active ingredients in, say, voltarol - an anti-inflammatory tablet used in joint and muscle pain - may be more or less the same between different brands, the price can vary enormously. For example, a generic version can cost around £4.50 for 84 50mg tablets, compared with a brand costing around £7 for 28 of the same dose capsules.

There are only rare exceptions where it really makes a difference to prescribe by brand. In treating epilepsy, doctors are taught to stick to the same brand of anticonvulsant as much as possible, because even minor fluctuations in strength can, Epilepsy Action says, lead to seizures, even if the person has been seizure-free for some time. So why do doctors prescribe one branded drug over another?

Sometimes there is only one drug of its type, as has been the case with clozapine up until now. Here, until recently, it didn't really matter whether you prescribed it as clozapine or by its brand name, Clozaril. There was only one option and you would be dispensed the same drug - until the competition came along.

Some pressures to prescribe a particular brand may come from patients. Even though the active ingredients in two tablets may be almost identical, patients may say they prefer one over another, (putting the doctor under pressure to prescribe their preference). The other pressure is from pharmaceutical representatives. A vast amount of effort is put into persuading doctors that one drug is better than another.

Pharmaceutical companies are also under pressure to recoup the money spent on research. The industry has only a few years to make money on a new drug before the patent on it is gone - typically around 10 years. But in the end, it is all paid for by the public - either through buying over-the-counter medication, or through prescriptions - via the NHS. According to the British Generic Manufacturers Association's website (www.britishgenerics.co.uk), the £1.3bn spent on generics in 2002 saved the NHS £4.6bn.

So if you end up getting plain packaged prescription drugs, with no logo and no branding, don't be offended. You are doing your NHS a favour.