The largest prospective study of adverse drug reactions (ADRs) needing hospital admission in the UK has exposed a huge burden on patients and costs to the National Health Service (NHS) and an urgent need to reduce them. Moreover, most of the reactions were predictable and preventable.

Over six months one in 16 of 18 820 patients were admitted to two NHS hospitals in Merseyside, north west England, because of ADRs, 80% as a direct result and in 2% with fatal consequences. Median patient age was 76 years and median hospital stay eight days, using an estimated 4% of hospital bed capacity and translating to an estimated annual cost of up to £466m (€706m/$847m) over the entire NHS.

Almost three quarters of the ADRs were classified as avoidable, one in six was a drug interaction, and many occurred in elderly patients. Most commonly cited were low dose aspirin (with or without other drugs), diuretics, warfarin, and non-steroidal anti-inflammatory drugs other than aspirin.

Prescribers must bear the responsibility for reducing ADRs, say the researchers, but other useful measures would be implementing recommendations in the National Service Framework for the Elderly and increasing pharmacists’ input in prescribing.

The observational study took place between November 2001 and April 2002 in a teaching hospital and a district general hospital serving 300 000 and 330 000, respectively. It included every patient aged ⩾16 years admitted over that time who was judged by careful scrutiny to have an ADR but excluded overdoses.

Previous similar studies are too old, too small, or too specialised to be useful.