The government's advisers took too long to recommend banning mephedrone, allowing dealers to build up a stash of the drug, its chief drug adviser says.
The first problems with mephedrone were highlighted in September 2009, but the drug was not banned until March 2010.
Professor Les Iversen said he was "fairly confident" that "a good deal of stockpiling was going on" between January and March 2010.
Mephedrone became a class B drug, the same as speed or cannabis, last April.
Referring to the banning of mephedrone, Professor Iversen said: "There's a danger here of not acting quickly enough."
He said the Advisory Council on the Misuse of Drugs would need to act more quickly with the planned new temporary banning orders which are currently going through Parliament.
"One of the points of having a temporary-class drugs order is that it's something that can be done in a hurry and put a clamp on the further escalation of the use of that particular compound.
"So if we take too long about our deliberations, as we probably did in the case of mephedrone, it gives the users and dealers an opportunity to buy and stockpile that drug during the period while it's still legal.
"So while we want quick action, we don't want to make a hasty decision that we regret later. There's a fine balance to be had there," he said.
Under new plans, the ACMD will have 20 days to consider an initial ban and then around a year to provide a full report of the associated harms.
Prof Iversen also warned the temporary bans would place a "heavy burden in terms of workload" on the ACMD and called for a research budget to be ring-fenced to enable them to carry out basic research into the effects of the drugs.
"It can be done on quite a limited budget, but someone needs to have a budget," he said.