By stopping or slowing the multiplication of cells, anti-cancer drugs stop or slow the growth of tumours. Some simply halt cell division; some damage DNA; others interfere with the cell's internal chemistry. All these drugs, however, act indiscriminately on dividing cells. So when damaging the rapidly growing cells in a tumour, they also damage healthy, rapidly growing cells. They affect the lining of the digestive system, causing a sore mouth and diarrhoea; they may damage the roots of the hair and so cause baldness; and they affect the bone marrow, where blood cells are manufactured, causing anaemia and bleeding disorders.
A decision to treat a patient with cancer by drugs should, then, be based on a cost benefit analysis that shows that the likely gains from treatment outweigh its inevitable unpleasantness and riskiness. A few types of cancer are treated by drugs as the first line of action, with a high chance of cure. Examples include some kinds of leukaemia and lymphomas such as Hodgkin's disease. In these circumstances patients are usually willing to put up with serious side-effects in the hope that they will survive.
Drugs may be given at the same time as surgery to remove a tumour, or shortly after the operation with the intention that they should knock out any microscopic traces of the cancer left behind and any small, symptomless clumps of cancer cells that may have spread in the bloodstream. This adjuvant chemotherapy has been shown to improve the chances of long-term survival for patients with breast and bowel cancer, but the cost-benefit equations are not so clear. Only a fraction of the patients treated by surgery will have tumour cells left behind, and only a fraction of those will be cured by the chemotherapy. As yet there is no certain way of distinguishing the patients who will gain from the treatment from those who will not, so it is offered to all. In these different circumstances, the amount of risk that a patient may be willing to tolerate is likely to be smaller than in the treatment of leukaemia.
The third main category of anti-cancer treatments is palliation for those patients whose disease is too advanced for a cure to be possible. In some cases all signs of the cancer may be suppressed for many months, but more often the remission achieved is only partial. What the patient should be offered is a realistic assessment of the best and worst case scenarios, taking account of the likely sensitivity of the tumour to drugs and their side-effects. Taking a decision not to give any anti-cancer treatment may be hard, but it sometimes opens the way for acceptance that death is inevitable.
Sadly, the anti-cancer drugs in current use show few real advances over those introduced in the 1960s and 1970s. Advances in our understanding of the genetic basis of cancer are leading to ways of switching off the switches (the oncogenes) that made the cells cancerous in the first place. Treatment of that kind should affect only cancer cells and so should have far fewer side-effects.