Banning the "dance drug" mephedrone may have cost lives rather than saving them – by driving users back to cocaine, an expert said yesterday.

Latest figures show deaths from cocaine and ecstasy fell during the first six months of 2009 at a time when the popularity of mephedrone, then still a "legal high", was rising. Separate evidence suggests that many drug users may have substituted it for cocaine, which could account for a decline in cocaine-related deaths.

Although mephedrone itself has been linked with several deaths, subsequent investigations have cast doubt on how dangerous it really is.

The ban on the drug was announced last March by former home secretary Alan Johnson after he received a recommendation from the Advisory Council on the Misuse of Drugs (ACMD). The decision followed a public outcry over the deaths of two teenagers – Louis Wainwright, 18, and Nicholas Smith, 19, from Scunthorpe – although toxicology tests later showed they had not taken the drug.

Professor Sheila Bird, of the Medical Research Council's Biostatistics Unit in Cambridge, said yesterday that after rising for four years, deaths from cocaine fell in the first six months of 2009 to 66, a statistically significant drop from 95 in the same period of 2008. Delays in reporting drug-related deaths mean the figures have only now become available.

Writing on the website Straight Statistics, Professor Bird said mandatory drug tests carried out on soldiers by the army showed a sharp fall in cocaine use during 2008 which continued into 2009. "I speculated that this decline might be the result of soldiers' shifting to mephedrone, then a 'legal high'. If that were true among the wider drug-using population, then the decision in April 2010 to make mephedrone illegal may have had the unintended consequence of reversing a notable decrease in cocaine-related deaths," she wrote.

But Professor Leslie Iversen, chairman of the ACMD, dismissed the suggestion as "purely speculative". "There is also evidence that mephedrone was being used by young people who had never used drugs before. The poor quality of cocaine in the UK may have led people to mephedrone – but that is also speculation. There isn't any hard data," he said.