Commonly prescribed medicines could be killing up to 10,000 people a year, researchers have warned.
A study by the University of Liverpool suggests that adverse reactions to drugs accounts for more than 6% of hospital admissions.
Researchers estimate these admissions cost the NHS £466m a year.
They have called for urgent measures to reduce the burden on the health system and improve prescribing practices.
The researchers studied 18,820 patients, aged over 16, who were admitted to two NHS hospitals in Merseyside during a six-month period in 2001-02.
They found 1,225 admissions were related to adverse drugs reactions.
Most of the patients recovered, but 28 died because of the reaction - many from internal bleeding caused by aspirin.
Aspirin, which is often prescribed to patients to avoid heart disease, was most commonly implicated - accounting for 18% of the admissions.
Other drugs which caused a bad reaction included the anti-clotting drug warfarin, non-steroidal anti-inflammatory drugs and diuretics.
The researchers estimated that adverse reactions causing hospital admission were responsible for the deaths of 5,700 patients a year in England.
Taking into account incidents occurring while patients were in hospital and admissions together, the figure could be greater than 10,000 a year.
They said that many incidents may be preventable through simple improvements in prescribing.
