Large quantities of legal highs are being stockpiled before a new law prohibits the sale of the drugs, leading Scottish charities have warned.
Users of the potentially dangerous "club drugs" are taking advantage of heavy discounts offered by online suppliers before they are banned.
Drugs workers claim it is putting the health of vulnerable addicts at risk.
The UK government said the Psychoactive Substances Act was expected to come into force this spring.
Kenny Malcolm, of Aberdeen-based Alcohol and Drugs Action (ADA), said: "We know of people who have thousands of pounds worth of these drugs in their houses.
"People have been stockpiling for the last six months because the legislation has been going through since last year."
The new law was due to be implemented on 6 April but charities claim the delay has given users the opportunity to "double stockpile".
The government will have to give 21 days' notice to the UK parliament before the legislation can be enforced.
It will make it an offence to supply any psychoactive substance - except nicotine, alcohol, caffeine and medical products - but possession will be legal.
Emma Crawshaw, the chief executive of Edinburgh-based drugs agency Crew 2000, said service users have admitted building up a reserve of the drugs.
"Unfortunately the effect of the implementation of the psychoactive substances act being delayed means there's going to be double stockpiling now," she added.
She said websites which sell the synthetic drugs are encouraging people to buy larger quantities ahead of the ban coming into force.
BBC Scotland has found online suppliers advertising a 20% discount on orders over £100, free next-day delivery on orders over £49 and "buy two, get two free" offers.
They describe their products as "research chemicals" which are "strictly not for human consumption".
Ms Crawshaw said: "It's a very, very dynamic market and it's certainly responding to the ban and leading to people stockpiling."
However, people who build up a reserve of their favourite legal high may find themselves at risk, she warned.
"There's a real danger of them starting to increase their use ever so slowly if they've got large supplies available to them," she said.
"If people are in a position where they're finding it difficult to control their intake, having more stuff in front of them is bound to be more of risk for them."
Are legal highs safe?
Officially known as a new psychoactive substances (NPS), these drugs are synthetic chemical substances, created in a laboratory to mimic the effects of legal highs.
Although "legal", drugs experts warn that they are not necessarily safe to use - in 2014 they were linked to 112 deaths in Scotland.
Synthetic cannabis - one of the most popular products - has caused particular problems, according to Alcohol and Drugs Action.
Mr Malcolm, of Alcohol and Drugs Action (ADA), said: "We're seeing people who are becoming very dependent extremely quickly - physically dependent, psychologically dependent within a couple of months.
"Where they're unable to stop, they are not able to sleep. When they do stop or reduce their use, there is physical problems, such as whole body pins and needles, stomach and leg cramps, headaches.
"People have had fits and seizures when they've stopped or even when they've been using."
David Liddell, CEO of the Scottish Drugs Forum, said the new law makes no distinction between drugs on the basis of level of harm.
"It will be legally possible, when the act is enforced, to possess substances which mimic the effects of banned substances - cannabis, amphetamines, ecstasy, LSD and vallium - but that may be more dangerous than the substances controlled under the Misuse of Drugs Act," he said.
The new legislation is expected to lead to the closure of so-called "head shops", which sell legal highs on the high street.
Mr Malcolm said that will make a difference in the short term but he added: "What my fear is that people will start dealing.
"They will start buying these drugs in bulk from abroad over the internet and they will set themselves up as dealers.
"So I don't think these drugs will go away - we'll have a hidden population and it will be difficult to figure out what's going on."
Mr Liddell, of the Scottish Drugs Forum, has called for a complete overhaul of the UK's drugs laws.
He said: "Stockpiling drugs will, for some users, result in them using more than they would otherwise and there are harms that can result from that heavier use in terms of overdose and health issues.
"In the event of having an amount of drugs that is not for fairly immediate use, there can be the temptation to supply others particularly if the new law results in a blip in terms of supply through normal routes."
The Home Office said the new law has been delayed while the government puts in place a programme of testing to demonstrate a substance's psychoactivity.
A spokesman added: "The landmark psychoactive substances act will fundamentally change the way we tackle these drugs and put an end to unscrupulous suppliers profiting from their trade. Our message is clear: offenders will face up to seven years in prison."