The number of people dying from now-banned legal highs rose sharply between 2009 and 2010, new figures have shown.
A report for the National Programme on Substance Abuse Deaths revealed 43 people in the UK died after taking now-outlawed methcathinones in 2010, compared with five in 2009.
The group includes mephedrone or "meow meow", which alone caused 29 deaths.
Overall, drug-related deaths fell by just under 14%, with Brighton having the highest rate, at 14.8 per 100,000.
The total number of deaths from drugs throughout the UK fell from 2,182 in 2009 to 1,883 in 2010.
The National Programme on Substance Abuse Deaths (NPSAD) said heroin-related deaths fell "significantly". In 2010, it was involved in 41% of all drug-related fatalities, compared to a proportion of 53% the previous year.
'Tightened their grip'
The annual report for NPSAD was compiled by the International Centre for Drug Policy (ICDP) based at St George's, University of London.
The team looked at drug-related deaths that had been formally investigated by the authorities, based on information from coroners, police forces, and the Northern Ireland Statistics and Research Agency.
The study concluded that former legal highs, such as meow meow, "tightened their grip on the recreational drug scene in western Europe but especially the British Isles".
It went on: "It is now difficult to gauge with any certainty what will be the next 'big thing' that will capture the attention of the experimenter or regular recreational drug user."
One of the report authors, John Corkery, said a dip in deaths from opiates such as heroin in 2010 could have been down to the popularity of mephedrone and other methcathinones.
He said: "They were thought to be less dangerous as they were promoted as legal. Of course, legal does not mean safe.
"Mephedrone is still causing deaths in 2012, and new substances are being identified all the time.
"These drugs are not tested, we do not necessarily know what the effects will be," he added.
While England, Scotland and Wales each saw a drop in drug-related deaths, Northern Ireland saw a slight rise - from 65 to 72.
The majority of deaths (72%) across the UK were among men aged between 25 and 44-years old. More than half of deaths (64%) occurred after an accidental overdose.
Professor Hamid Ghodse, director of the ICDP, welcomed the general fall in drug-related deaths but warned that finding "prevention strategies" is still a priority.
"It should not make us complacent as there are indications that there is still a general upward trend in fatalities involving emerging drugs such as mephedrone and prescription drugs such as methadone.
"This is a great concern and it is clear that much work is still required in improving access to effective treatment and rehabilitation services and most importantly, finding prevention strategies to stop people being at risk in the first place," he said.
Leading anti-drug charity Addaction also welcomed the drop in deaths, but said remained concerned about the developing use of less "established" drugs.
Simon Antrobus, the charity's chief executive said: "We need to acknowledge that drug use in the UK is changing.
"Developments, such as the ability to buy drugs over the internet, mean that some of this change has been at an astonishing rate.
"Some of the deaths reported will relate to substances that have only appeared in the past couple of years, rather than to 'established' drugs such as heroin.
"We see the use of these 'legal highs' with an increasing frequency in our services, and are concerned at the further damage their long-term use will cause."
Methcathinones were classified as class-B drugs in 2010, losing their "legal high" status.
The government announced on Thursday that two new legal highs, "black mamba" and methoxetamine or "mexxy" are to be criminalised.