Cannulas are used to help take blood and to give drugs and fluids
The insertion of tubes into hospital patients' veins is unnecessary in a third of cases, pharmacists have warned.
Researchers from Aberdeen's Robert Gordon University said this needlessly exposed them to serious complications, such as infections and blood clots.
Just under 350 patients were studied over six weeks, the majority of whom had the tubes, called cannulas, fitted.
An A&E expert recognised cannulas should be used less frequently and for
The study was presented to the British Pharmaceutical Conference in Manchester.
Cannulas - hollow plastic tubes with a needle at the tip which cost around £1.70 each - are used to give medication and fluids to people who cannot swallow because they are unconscious or being given nil by mouth, and it has been estimated that around 80% of hospital patients have them fitted.
Drugs may also be more easily absorbed if given this way.
But potential complications include problems with veins (phlebitis), drugs leaking into tissues around the site of the tube, serious infection and blood clots.
Of the patients studied - who were all treated in the acute medical assessment unit of Aberdeen Royal Infirmary, 91% of patients had a cannula inserted. But 28% of the tubes were never used.
The researchers also found that in 71% of patient records there was no documentation of a cannula being inserted, while in 57% there was no documentation of it being removed.
Four patients had developed blood poisoning, which infection control specialists said was likely to be linked to the cannula.
The researchers, led by Dr Yash Kumarasamy, said that in many UK hospitals, it has become common practice to insert an intravenous cannula when the patient is admitted, irrespective of need.
He said: "We would like to see the introduction of a formal procedure under which hospital pharmacists review patients and their medications and make recommendations to the treatment team about whether or not a cannula is needed."
Dr Martin Shalley, a former president of the British Association of Emergency Medicine, agreed there had been an over-reliance on cannula use.
He said many trusts had policies saying cannulas had to be removed after 72 hours
"It used to be a knee-jerk response to insert a cannula - but we now recognise there's a need to think if fitting one is a benefit for that patient.
"It's entirely reasonable to look at their use. That's the case in A&E medicine and across acute medicine too."
And Dr Shalley said he thought the level of use of cannulas had increased the level of hospital-acquired infections such as MRSA.