In a review of the evidence, a team of European researchers on Thursday scoffed at the notion that erythropoietin (EPO) gives cyclists an edge.
And they pointed out that the drug has many perils for those who use it illicitly, including blood clots that can cause strokes and heart attacks.
"Athletes and their medical staff may believe EPO enhances performance, but there is no evidence that anyone performed good experiments to check if EPO would actually improve performance in elite cyclists," said Adam Cohen, a professor at the Centre for Human Drug Research in the Netherlands.
EPO is a natural hormone, produced in the kidneys, which helps regenerate the red blood cells that transport oxygen around the body.
A man-made version of the hormone is licensed for treating renal patients to help them combat anaemia.
As early as 1990, the drug appeared on the International Olympic Committee's banned list, given the suspicions that by increasing red-blood cell mass, it also boosts exercise capacity.
EPO then engulfed professional cycling, breeding a scandal that erupted this year when Lance Armstrong was stripped of his record seven Tour de France titles.
But, said the new study, no solid evidence exists to back the belief that EPO is as effective as certain trainers and team doctors believe.
And, it added, there are plenty of reasons to say the faith is as dangerous as it is misplaced.
Cohen's team trawled through published studies that tested EPO's effect on healthy cyclists but found none which had participants of competition level, whose genetic profile and training programmes differ from those of sub-elite groups.
The big belief behind EPO is that it improves maximal oxygen uptake – known in scientific parlance as VO2 max – and thus boosts power output.
But Cohen found no proof for this, at least as far as professional cycling is concerned.
In tests, cycling volunteers were usually assessed for VO2 max for just 20 minutes or so, a far cry from a five-hour grind of a cycling race.
In any case, said the study, VO2 max is only a minor factor in the performance of endurance cyclists.
Only small segments of professional cycling races are cycled at such severe intensities that VO2 max is decisive. There are many other factors, none of them influenced by EPO.
They include the blood lactate threshold, which determines the point at which muscles tire; work economy, which is the efficiency of the metabolic system to convert energy into movement; increased cardiac volume; and the quantity of muscle mass available for sustained power production.
"There is no scientific basis to conclude EPO has performance-enhancing properties in elite cyclists," is the blunt conclusion of the study, which appears in the British Journal of Clinical Pharmacology.
"Additionally, the possibly harmful side-effects have not been adequately researched for this population, but appear to be worrying at least." – Sapa-AFP