A fifth of patients on an intravenous drip develop complications because they are given the wrong levels of fluid, according to a review of guidance in England and Wales.
Too much fluid can cause heart failure and too little leads to kidney problems.
The National Institute for Health and Care Excellence (NICE) said doctors and nurses needed better drip training.
Patients’ groups said the scale of the problem was “staggering”.
Thousands of people each year need a drip in hospital. But NICE warns that staff are putting lives in danger due to a lack of education in managing intravenous drips.
It has developed new guidelines for the NHS in England and Wales.
Dr Mike Stroud, a gastroenterology consultant at Southampton University Hospitals NHS Trust, who developed the guidelines, said: “Doctors and other health professionals are not well educated in terms of what a patient needs and that is astonishing really.
“This needs to change.”
Hospitals will also be expected to appoint an “intravenous fluid champion” and patients’ drips will need to be managed and monitored more closely.