Behind the scenes of dancefloor euphoria in clubs aross the country is "an endless cat-and-mouse game" between criminal drug producers and the police.

It is a game in which the criminals are gaining ground.

It had seemed the widespread use of ecstasy, the drug which fuelled the rave scene 20 years ago, had faded, replaced by other stimulants including alcohol and cocaine.

But the tragic death of 17-year-old Regane MacColl in a Glasgow club last weekend suggests ecstasy is back in new and dangerous forms.

Doctors have reported a recent rise in the numbers of patients arriving at A&E units after taking ecstasy, with one major hospital in Glasgow now seeing up to 10 people in every weekend, some as young as 15.

And analysts suggest ecstasy production is recovering from a lull brought about by various factors, including a crackdown by China on smuggling one of the main ingredients used to make ecstasy, or MDMA.

Laurent Laniel, scientific analyst at the European Monitoring Centre for Drugs and Drug Addiction, said the availability of MDMA had dropped between 2009-11.

But he added there were signs ecstasy production was now picking up again in a "big way", with manufacturers sourcing other ingredients to make the drug.

He said: "There have been seizures or dismantlement of very large labs manufacturing MDMA, especially in Belgium and the Netherlands, which is where ecstasy manufacturing is concentrated in Europe.

"Every time they find a new substance to make their drugs, we control it and make it difficult for them to procure it, but then they jump to another one.

"They have become more professional and they probably employ chemists or people who are very conversant with chemistry.

"Now some guys are really very good at what they are doing and very difficult to control.

"It is like an endless sort of cat and mouse game'.

Experts says the new supply is worrying because it includes super-strong tablets.

After Regane MacColl's death, police issued a warning over the Mortal Kombat tablet, which she is believed to have taken and which was linked to four other people becoming ill in separate incidents in Glasgow and Ayrshire.

The 17-year-old, from Duntocher, West Dunbartonshire, fell ill at The Arches club in Glasgow and died in hospital in the early hours of the morning.

The dangers of the new strains of ecstasy are increased when combined with a growing trend among clubbers of topping up ecstasy with alcohol or other drugs.

Dr Richard Stevenson, a specialist in emergency medicine at Glasgow Royal Infirmary, said they had noticed an increase since the start of December in people turning up at casualty after taking ecstasy on Friday and Saturday nights.

He said: "At the weekends we are seeing multiple presentations a night - anything between one and five. All have been drinking alcohol and more worryingly, some are using additional drugs as well - such as cocaine - which is a time-bomb."

He added: "The spectrum of use is also changing, so we are seeing a shift towards much younger people presenting having taken ecstasy - 15 is the youngest we have seen here."

Stevenson said people were commonly brought to casualty by police or ambulance suffering a fast heart rate and were usually fine.

But he added: "There are other people found wandering the streets or lying in their own vomit or collapsed in clubs, and when they come in to us they are confused - they don't know where they are, they don't know what they are doing.

"They are agitated, have quite fast heart rates and they have got that teeth-grinding, where they grind the back of their teeth constantly.

"That is the early stages of warning signs of ecstasy toxicity but then it starts to become a problem when their body temperature starts to rise.

"We can intervene at that stage, but once the body temperature rises to above 41ÚC, that is when we struggle to be able to treat people." Then, he says, the chance of dying becomes very high.

Katy MacLeod, national training and development officer at the Scottish Drugs Forum, also pointed to users of ecstasy mixing it with alcohol or other drugs: "It is becoming more normalised to take multiple substances together," she said.

"If you look to the early years of ecstasy it wasn't as common for people to drink alcohol at the same time, or if they did, it was just one or two drinks.

"I've noticed a bit of a shift in attitudes to ecstasy and alcohol and combining it with other drugs. I think people maybe aren't as aware of the risks of what that can trigger.

"If you are drinking alcohol as well, for example, the risk of dehydration goes up."

In the 1990s, the most high-profile ecstasy death was Leah Betts, who died after taking the drug at her 18th birthday party. A picture of her on a life-support machine became a defining image for anti-drugs campaigns.

Despite high-profile deaths, figures published for 2010-2011 show ecstasy was the third most popular drug among people aged 16 to 24 in Scotland, after cannabis and cocaine. Around 8% of males in this age group and 2% of females reported using the drug.

The latest available figures show there were nine deaths linked to ecstasy in Scotland in 2012, compared to none in 2010. Last year, a major alert was issued over a batch of fake ecstasy pills, known as Green Rolex. It contained para-methoxyamphetamine (PMA), a chemical which can be five times stronger than MDMA. The deaths of seven people in Scotland were linked to the pills.

Toxicology results on the Mortal Kombat pill linked to Regane MacColl's death are still to be revealed, but sources said early indications suggested PMA was not involved.

MacLeod added: "What is important to get across is sometimes young people can think problems are down to just a rogue batch [of drugs]. But there are risks with taking ecstasy, even if it is MDMA."

Carole Kelly, chief executive of drugs charity Crew, said: "I don't think you ever know what is in a pill these days - that is part of the concern.

"Mortal Kombat were around last year and didn't appear to be causing the same reaction they are causing in this current batch."

Last month, Wales became the first part of the UK to offer a Government-funded drug testing service. The Welsh Emerging Drugs & Identification of Novel Substances project - known as Wedinos - allows anyone to send a sample of a drug in for laboratory testing anonymously, with the results posted online and identified through a reference number.

Drug-checking services already exist in other countries such as the Netherlands,

Sophie Macken, director of the Independent Scientific Committee on Drugs, said: "It is not only of great value to anyone who does decide to take drugs - because they can make a more informed decision - it is also really useful for health and drug services and for the Government.

"You have a real-time report of what people are taking and if something like PMA does crop up you can take much quicker action. That is something that could potentially work in the UK without having to resort to huge changes in the drug laws."

Kenny Simpson, a drugs expert at Police Scotland's STOP unit, said issues around "quality control" of ecstasy ranged from those containing in excess of 200mg of MDMA - around a 30-50% increase in strength compared to previous years - to pills containing other controlled drugs.

HE added: "It is down to the organised criminal group on how they produce them and what sort of expertise they develop.

"We have concerningly identified ecstasy-tablet manufacture in Scotland where the tablets have been pressed using small pill-making machines, but these are capable of producing thousands and thousands of tablets.

"In the Netherlands they hold a degree of expertise in their ecstasy production, while Scotland as an ecstasy producer hasn't got that longevity of expertise."

Simpson said work was currently under way with licensing authorities to draw up new guidelines for clubs and music-event organisers on how to deal with drugs on their premises, to be published by summer.

"One of the concerning trends is that people pre-load before they go into premises, so for a drug such as ecstasy it can take 45 minutes to take effect," he said. "You get through the door right as rain and then all of a sudden you have a reaction.

"It is not just all about how they turn up at the door."

The Arches cancelled club events for two weeks after Regane MacColl's death, which it said was "both as a mark of respect, and to enable us to work closely with Police Scotland in reviewing the situation".

One 18-year-old regular at The Arches who was there last Saturday said: "You see ecstasy everywhere - it is not as if it is one place, anywhere you go it is happening. I don't take it myself, but it is really common.

"To be honest I wasn't shocked [at the ecstasy death], more that she was underage.

"Last Saturday I got asked for my identification twice going in to The Arches and they don't let you in without being searched."

The clubber said Regane MacColl's death was likely to be an "eye-opener" for a lot of people, but she added: "Is it going to change people's behaviour? Probably not."