Thousands of drug injection errors are probably made every day in NHS hospitals, say researchers.
When nurses give drugs intravenously, they are making mistakes in almost half of the injections because they are poorly trained, the study says.
Some of these are putting patients at risk. At least one patient each day in every major NHS hospital may experience a "potentially serious error".
Researchers carried out their study in just two hospitals, but say their findings are likely to be reproduced across the health service.
They recommend nurses are given better training and technology to help them give intravenous (IV) drugs properly. They add that the amount of preparation of the drug needed on the ward should be reduced to cut the risk of errors.
The team observed the preparation and administration by nurses of IV drugs, which are injected into the vein, over six to 10 consecutive days on 10 wards in the hospitals.
Just over 100 patients were given 430 doses of intravenous drugs during the period.
Errors which might not matter with other patients could be very serious in these cases
Professor Nick Barber, University of London
Errors were seen in 212 doses. In around a third, they were potentially harmful, and in three cases, the error was potentially severe, and could have led to long-term hospitalisation or even death.
However, the observer in the study who was a trained pharmacist intercepted to ensure the patient was not harmed.
Mistakes in the administration of drugs occurred in a third of cases, preparation errors, where there could be several steps preparing the drugs, occurred in 7%.
The most common errors made were giving concentrated doses too quickly and mistakes in preparing drugs that required multiple steps, such as diluting or mixing them with a solvent.
The researchers said the rate of error they observed was high.
Nick Barber, professor of the practice of pharmacy at the University of London, told BBC News Online: "This is something that concerns me, and I think it's something that needs acting on by hospitals.
"For people who require IV drugs, it can be all that's keeping them alive. So errors which might not matter with other patients could be very serious in these cases."
He said that a lot of problems occurred when people did not follow the rules.
"It's like when people break the speed limit. In a lot of cases, people can break the speed limit and it's perfectly safe. But on some occasions, things go wrong."
He added: "Sometimes, the training given is not sufficient. We need to increase the amount of training on the making up of drugs as well as information on dilution."
Professor Barber said he hoped the recently formed National Patient Safety Agency would be able to make a difference.
A spokesman for the Royal College of Nursing said: "This study only covered two hospitals and a very short period of time."
But he added: "There are improvements which could be made in the training of nurses, and the role pharmacists play in the training of nurses."
He said nurses were not currently told about how to give IV drugs in their first few years of training, and were only given instruction when they moved onto a ward where they were given.
A Department of Health spokeswoman said: "The NHS is committed to making drug treatment as safe as possible.
"Standards of prescribing in this country are high and the vast majority of drug treatment is provided safely.
"However mistakes do occur. They can arise in the prescribing, dispensing or administration of medicines, and the consequences can be serious."
She said Chief Pharmaceutical Officer Dr Jim Smith, and the NPSA were preparing a comprehensive report on medication errors which would provide guidance for health professionals.