Drugs are supposed to make us better, but sometimes they actually harm us.

The word 'pharmaceutical' derives from the Greek 'pharmakon', which means 'medicine' or 'toxin'. These might seem like complete opposites, but in fact medicines and toxins are very similar - they both interfere with our body chemistry. And a number of medicines have been developed from toxins, such as the heart drug digoxin.

Any compound that interferes with our internal biochemistry can have powerful effects. All medicines have side-effects, but they are used because their benefits outweigh their harmful effects. The safety of medicines is assessed by extensive testing and clinical trials. But clinical trials cannot eliminate the risks and, unfortunately, some people will be made ill, perhaps even killed, by their medication.

Adverse drug reactions (ADRs) are surprisingly common. A study carried out of nearly 20 000 admissions to hospital in Merseyside in 2004 found that 1225 (6.5%) were due to adverse drug reactions. They accounted for 4% of hospital bed capacity and the annual cost to the NHS of ADRs was estimated to be £466 million.

This suggests that, at any one time, the equivalent of up to seven 800-bed hospitals may be occupied by patients admitted with ADRs.

The figures also suggest that ADRs leading to hospital admission cause around 5700 deaths every year - so all fatal ADRs, including those occurring while patients are in hospital, may exceed 10 000 a year.

It is not experimental new drugs that are the main problem. The biggest cause of hospital admissions is aspirin, which can cause bleeding in the gut. Reactions to diuretics and the blood thinner warfarin were also common.

ADRs are thus a major health problem - in fact, one of the leading causes of death in the country. This is why there is such an the intensive research effort to identify the genetic factors linked to adverse reactions, and so prevent patients from receiving drugs that might seriously harm them.