Bath oils used to help treat eczema in children offer no meaningful benefit as part of their care, a trial has found.
Emollient bath additives are estimated to make up as much as a third of the cost of treating eczema in the UK.
But a new trial, published in the BMJ, found "no evidence of clinical benefit" when they were used in addition to other treatments for the condition.
Experts said it suggested the £23m spent each year by the NHS on the additives could be used better.
Eczema is the most common inflammatory skin condition in childhood and often lasts for years.
Emollients come in three forms – leave-on emollients, soap substitutes and bath additives – and are often prescribed in combination with each other.
Although there was evidence that leave-on emollients and soap substitutes were effective, until now there had been a lack of strong research on how well bath additives worked, authors of this new paper said.
For their study they randomly assigned 482 children aged one to 11 from England and Wales into two groups – with one receiving the bath emollient and the other not using it.
All of the children continued their normal eczema care routine, including regular use of leave-on emollients and corticosteroid creams, which reduce inflammation and irritation.
Symptoms had improved in both groups over the course of 16 weeks, but there had been no statistically significant difference between them, the trial found.
There were also no significant differences between the two groups across other measures, including eczema severity over one year, number of eczema flare-ups, quality of life and cost-effectiveness.
'Pouring stuff down the plughole'
Dr Miriam Santer, a GP and associate professor at Southampton University in primary care research, who led the study, said: "We don't need to tell people to put the bath additives in the water anymore.
"That will save trouble for families, knowing how best to treat the eczema and which treatments really help, and will also save the NHS money.
"The bath additives don't work – basically you're pouring stuff down the plughole."
But she added that people should continue to use the leave-on emollients and soap substitutes.
Dr Martin Ward Platt, a consultant paediatrician in neonatal at the Royal Victoria Infirmary in Newcastle, not involved in the study, said the trial's "robust" findings suggested NHS money should be spent on other treatments.
He said: "That doesn't mean they shouldn't be available on sale for people who want to give them a try or use them out of personal preference, but the notion that one should prescribe them and spend public money on them is heavily undermined.
"There's a good case now to not have these on prescription and use the money elsewhere."
The study said it could not rule out the possibility that bath additives could offer small benefits to children bathing more than five times a week or among children under five.
One limitation of the trial was that the bath additive was used with other treatments, so it did not look at how effective it was in isolation.
But Dr Ward Platt said it was "very unlikely" that it would work well on its own, as if it did it would probably have added to the effect of the other treatments.