The revolutionary drug Herceptin was this week licensed by the European Commission to fight early-stage breast cancer, and it now seems probable that it will shortly be cleared for clinical use in the UK. This will be a huge relief to many cancer sufferers, some of whom have conducted prolonged public campaigns to access the drug on the NHS (at an estimated cost of £20,000 per person per year, or £100m in total, or one-quarter of the total national bill for cancer drugs). One sufferer even won an Appeal Court ruling to establish her right to the drug. Herceptin has become the arch- example of the NHS postcode lottery, something we all hate.

Studies in the US show that the use of Herceptin, along with conventional chemotherapy, reduces the mortality rate of users, three years after treatment, by 33%, and reduces the cancer recurrence rate by more than 50%. Put like that, it is scarcely a surprise that so many breast cancer patients are demanding access to the drug on an accelerated basis. In their shoes, I'm certain I would have done the same.

But before we get carried away with what has been called a "miracle drug", let us examine those statistics a little more carefully. About 40,000 people are newly diagnosed with breast cancer in the UK each year. Of these, around 10,000 have the type of tumours (known as HER2 positive) on which Herceptin works. So the drug is sadly irrelevant for 75% of breast cancer patients.

For the remainder, US studies suggest that the three-year, disease-free survival rate will be 75.4% for those on conventional treatment, and 87.1% for those on Herceptin. Subtracting these numbers from 100, we obtain the advertised 50% improvement in disease-free survival rates (from 24.6% to 12.9%). But another way of looking at it is that about 12 out of every 100 users of Herceptin, or about three out of every 100 breast cancer patients, will be helped by the drug to remain disease-free after three years. This sounds less dramatic, though of course it is still very worthwhile.

Moving to life expectancy, Herceptin improves the chances of survival after three years from 91.7% to 94.3%. So the reduction in the death rate is indeed one-third (from 8.3% to 5.7%). But in less dramatic language, Herceptin keeps about three people out of every 100 alive after three years for those who take the drug. This amounts to less than one extra person per 100 among all breast cancer patients.

I am emphatically not arguing that the drug is ineffective, or that it is too costly to prescribe in the NHS. But what I am arguing is that it has been too easy to conclude from sensational press stories on the drug that its benefits will be more widespread than they might actually be. This has greatly increased the political intensity of the issue, and the distress for those patients who have been unable to get hold of the drug.

One very intelligent breast cancer sufferer, Lisa Jardine, appeared on Radio 4 on Sunday to argue that "for women like myself, the new drug seems to promise a smallish reduction of an already lowish chance" of the disease recurring. She is absolutely right.