Image caption Leslie has taken pills to Nass festival that are stronger than he has ever used before

Ecstasy, or MDMA, is becoming more readily available to buy, particularly on the so-called "dark web". And this has seen the drug grow in popularity, and in strength.

Leslie - not his real name - is 18, and has come with friends to Nass festival in Somerset - an event that celebrates music, skating and BMX riding.

It has a zero-tolerance policy on drugs, but it is clear to see that this is being defied. The drug of choice appears to be MDMA.

Leslie tells the BBC's Victoria Derbyshire programme he has 24 pills with him, which he bought for £85 on the dark web - a hidden area of the internet accessible only through certain browsers.

Image caption BMX contests are held as part of the festival

In May, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) reported that high-strength tablets and powders were becoming more easily available, and Leslie's are the most potent he has ever taken.

He says it is the first time he has ever been scared of what he is using.

"Just when I was walking to one of the sets, it suddenly hit me and I was off my face," he says.

"Normally, I react well - I'd be happy, just really excited."

Find out more

The Victoria Derbyshire programme is broadcast on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.

Watch James Longman's full film on ecstasy and MDMA here.

Festival organiser Ryan Matthews is clear that "if people are caught with any illegal drugs on site, they are ejected".

But there are other worries too, of which Leslie is well aware.

"You're taking the word [of] the dealers themselves," he says, regarding the content of the pills.

Many illegal drugs are diluted with other cutting agents - in some cases, horse tranquilliser or rat poison.

About 10% of ecstasy users buy the drug on the dark net, according to a sample of 50,000 people by the Global Drug Survey, and Leslie believes this is the "smart" choice.

Without a middleman, he says, the purity of the drug is maintained.

"Stupid people will get dirty stuff from the streets," he adds.

'I'm a mother no longer'

The devastating effects of ecstasy, however, are well documented.

There has been a fourfold increase in British female clubbers seeking emergency medical treatment in the past three years, according to the Global Drug Survey 2016.

Michelle Shevlin's 22-year-old daughter, Stephanie, died in June after taking a pill in a nightclub.

"When I found out MDMA was one of the causes of her death, I was really angry, upset, very annoyed," she says.

"I still am annoyed."

Image caption Michelle Shevlin is finding it difficult to come to terms with her daughter's death

"It's hard to understand how someone can be here one minute and not the next.

"I do cry - I cry to sleep, I cry when I wake up, I cry just cleaning the house.

"It doesn't get any easier, and the days at the moment seem to be getting longer and harder."

Ms Shevlin has a clear message for the dealer who supplied the drug that caused her daughter's death.

"I just hope that person is losing sleep every night they're alive... because I'm a mother no longer," she says.

'Lottery' of illegal drugs

The single pill that ultimately killed Stephanie was almost certainly weaker than just one of the three or four pills Leslie took at the festival.

And this is why Trevor Shine, from Tictac Drug Identification, believes that taking illegal, unregulated drugs is a "lottery" - with people's bodies affected in different ways.

"One person could just as easily die from a lower purity, while someone else could survive a higher one," he says - pointing to factors such as body size and alcohol or other drugs consumed at the same time.

He measures the strength of Leslie's drugs, and finds they are worryingly pure.

The ecstasy tablet he examines contains 233mg of MDMA, whereas "traditionally, the average dose in an ecstasy tablet would have been 85-115mg".

Image caption Trevor Shine believes the strength of Leslie's pills is dangerous

Mr Shine warns users of the dangers of taking the same amount of pills when the strength is so much purer, as they would with a lower dose.

It can cause serious health risks.

Prof Philip Murphy told Newsbeat the rising strength of MDMA could cause users more severe mental health problems.

Other class-A drugs - in particular cocaine and crack cocaine - have also seen a recent increase in strength, according to Tictac pharmaceutical analyst Anca Frinculescu.

'Extremely lucky'

After the festival, we reconvene with Leslie to discuss the findings.

He is unable to remember his evenings at the festival, but knows - on the night we met him - he had to take "quite a few more" tablets to replicate "the same effects of the first one".

He appears visibly moved when told of Ms Shevlin's loss of her daughter.

"It's scary," he says, "I'm extremely lucky."

There is BBC Advice here on ecstasy and MDMA.