An experimental drug probably does not stop the progression of the brain disease vCJD, experts have concluded.
However, the Medical Research Council said further large scale tests on animals should be carried out.
The MRC monitored the effect of Pentosan Polysulphate (PPS) on seven patients with vCJD or other degenerative prion diseases.
It conceded that the drug had appeared to help several people live longer than expected.
Several patients have taken the drug, including Jonathan Simms, now the UK's longest-surviving vCJD patient.
"We have got to look at any potential therapy most carefully"
There is no cure for vCJD and the majority of people who have developed the disease have died.
So far 111 people are known to have died from the disease in the UK, with another 45 probable fatalities. It is thought that five Britons are currently living with vCJD.
The MRC did not originally include PPS in a trial of possible treatments on the advice of the Committee on the Safety of Medicines.
However, the Simms family, from Belfast, won permission from the High Court to give Jonathan the drug in December 2002.
Since then it has been given to others, including Holly Mills from North Yorkshire, who started treatment within weeks of being diagnosed with vCJD in October 2003.
Her condition has been stable for 18 months.
Into the brain
The drug must be surgically administered directly into the ventricles of the patient's brain.
Lead researcher Professor Ian Bone accepted that his findings were not conclusive, as they were based on a small number of patients who were treated in different ways.
However, he said any clear benefits of the drug would have been revealed by the study.
Professor Bone, a consultant neurologist at Glasgow's Southern General Hospital, recommends more research in animals into how PPS penetrates and spreads through the infected brain.
He also said newly diagnosed patients should be fully informed of PPS when treatment options are discussed - and given the option of taking the drug.
The study also found little evidence of side effects from the drug. However, there were risks associated with the surgery required to deliver the drug into the brain.
Scans
Prof Bone said: "The drug does not appear to halt the progression of the disease.
"Loss of brain function continues after treatment has started and, where measured by imaging, loss of brain tissue also continued.
"The patients treated with PPS appear to have survived for unusually long periods.
"There is no question that PPS has worked for my daughter"
"However, we cannot conclude with certainty that the treatment has a beneficial effect, because it was impossible to make direct comparison with similar but untreated patients.
"Moreover, with such small numbers the results might be a matter of chance."
Gill Turner, from the CJD Support Network, welcomed the study, and said the findings were not surprising.
"We would support the call for more research. We have got to look at any potential therapy most carefully," she said.
Peter Mills, Holly's father, said the fact that Professor Bone had recommended more research was very significant.
He said: "There is no question that PPS has worked for my daughter. Holly would not be here now if we had not chosen to give her this drug."
A Department of Health spokesperson said it would work with the MRC to take forward the recommendations in the report.
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