Patients are dying and being seriously harmed because many doctors lack the sufficient knowledge to prescribe drugs properly, leading experts say.
The teaching of prescribing needs to improve, doctors say
Top doctors said the problem was being caused because the General Medical Council was placing less emphasis on pharmacology in UK medical schools.
They also said the risk was being compounded by the use of more complex medicines in the health service.
The GMC said it rejected the allegations completely.
The NHS spends £7bn a year on drugs, with an average of 12 prescriptions being written for each patient.
But the leading medics warned most of a doctor's knowledge about how and what to prescribe was learned "on the job".
They said a change in guidance to medical schools in 1993 by the GMC meant that there was less emphasis on prescribing skills.
The paper, Tomorrow's Doctors, changed the requirements on medical schools, allowing them more freedom to develop their own curricula.
Professor Sir Mike Rawlins, professor of pharmacology at Newcastle University, who is also chairman of the National Institute for health and Clinical Excellence, (NICE) said: "A great deal of mis-prescribing is because of a lack of knowledge. About 80% of adverse drug reactions are avoidable.
"In the most serious problems, people suffer and a proportion die because drugs are not used properly.
"This issue needs to addressed."
Sir Mike said there needed to be clearer advice about what schools should be teaching.
"Students need to be told about what drugs to prescribe, in what proportions and how they interact and then they need to be examined on it."
And he backed the recommendation made last week by Sir Liam Donaldson, the government's chief medical officer, that responsibility for education standards be stripped from the GMC.
Deaths due to adverse drug reactions have risen by over 500% since the early 1990s and are now estimated to cost the NHS £500m a year.
Professor Jeffrey Aronson, from Oxford University and president-elect of the British Pharmacological Society, said: "I think that a lot of this is actually preventable.
"I think that any dangers that occur could be prevented by careful prescribing, by careful use and by increased knowledge on the part, both of doctors and nurses and pharmacists who are prescribing drugs, and the patients who are using them."
But the GMC said: "We refute the suggestion that medical undergraduates are failing to learn to prescribe properly.
"It is clearly stated in our guidance that medical students must be taught to prescribe safely and effectively, and we regularly check medical schools to ensure they are following our guidance."
And a spokesman for the Department of Health said: "The reform of junior doctor training, Modernising Medical Careers last August means that all junior doctors now have to prove they are skilled in prescribing before they are able to move up to the next level of training.
"Skills junior doctors must demonstrate include being able to prescribe drugs appropriately, take accurate patient drug histories and recognise the sources of medication error and ways to minimise it."