The families of two young people who died after taking the fat-burning pill DNP are campaigning for it to be classified as a class-C drug to criminalise its possession and supply in the hope of preventing more deaths.

At least four people have died in the last 18 months in the UK after taking DNP, a fertiliser packaged in capsules and sold online to dieters and bodybuilders. More than 60 deaths have been officially attributed to the drug worldwide.

Lesley Mapletoft, the mother of Chris, 18, who died in June the day after taking his last A-level, said in an interview with the Guardian at her home in Twickenham, south-west London, that class-C classification would have deterred her son.

"Chris was not a wayward child," she said, speaking publicly for the first time. "I'm sure he would not have taken it if he knew it was an illegal drug. He was very against drugs. He didn't smoke, didn't really drink." He was, she said quietly, the perfect son.

The Houstons, parents of Sarah, who was studying medicine at Leeds University, also believe classification would have deterred their daughter, who died in September 2012. The two families have met MPs and the authorities to press for the change in the law.

Others whose deaths have been attributed to DNP are Sarmad Alladin, an 18-year-old creative arts student and bodybuilder who died last February, and Sean Cleathero, a 28-year-old father of a young child, who died a month later.

Currently, DNP (2,4-dinitrophenol) can be bought and sold legally as a fertiliser. The Medicines and Healthcare Products Regulatory Authority (MHRA) has no power to prevent its use as a drug because it is not a pharmaceutical.

Theoretically, the Food Standards Agency (FSA) can take action if DNP is sold for human consumption, but as the horsemeat scandal showed, it can struggle with enforcement against overseas suppliers.

The FSA has begun an education campaign, urgently warning the public not to take any tablets or powders containing the chemical. It says it is working with the police and local authorities to stop illegal sales from websites and to remind traders of the penalties for supplying the chemical in the UK.

The police can pursue anyone who supplies DNP specifically for human consumption in the UK and have charged four people in Cleathero's case, three of them with manslaughter.

But blocking access to DNP to stop more such deaths appears much harder. So desperate are people to lose weight that websites selling diet pills are widespread, even though the drug industry has failed to create a safe, effective and acceptable medicine.

Many sites sell DNP, although it is not uncommon to discover warnings such as "the excessive and improper use of DNP can lead to death!" on sites where it is sold in £30 or £50 batches.

On one site, a user writes: "DNP is pure hell … I've cycled it 8 times since 2006. As soon as the heat kicks in, the nausea sets in, and the lethargy kicks my ass … Body temp can spike as high as 105-108 degrees … any higher and your brain will fry and you'd better be prepared to make funeral arrangements." But, the writer added: "I know it's working."

Chris played rugby for his school in Hampton, usually as centre in the first XV. But his family do not believe his sport was the reason for buying the pills. It had more to do with the post A-level summer break in Ayia Napa he and his friends had planned.

"He was a good sportsman, but this was never about rugby, I'm sure," said his mother. "This was about a whole gaggle of them going off for their first summer holiday, having worked really hard – he got an A*, A, B for his A-levels, so would have gone to Bath University [to study business].

"I think this was about him looking perfect on the beach. And he was a good-looking lad. He did go to the gym, probably four times a week for an hour and he did lift weights, but I would never call him a bodybuilder.

"I just think he thought: 'You know, I'm looking really good, I'm on top of my rugby game, I'm working really hard, I'm going to Bath, I'm going to get my head down and work really hard.' Which he did. He was very focused. What a waste."

In his final year at school, Chris wrote an extended essay on nutritional supplements and their effects on the body. Although his family do not know if that was how he first learned of DNP, his computer showed he had researched the chemical before buying it.

"Chris was not a daft kid. He was not stupid. He wasn't reckless. He was considered and he'd done his research," his mother said. "He looked at DNP and I think he just thought: 'I'm 18. I'm invincible.' And it was just the worst judgment he could have made.

"He had his whole future in front of him. He was very loved. We've got books and books of the things people have written about him. He was really modest, very unassuming, talented."

The Mapletofts talked to Chris's friends immediately after his death to warn them about DNP and urge anybody taking it to stop and get medical attention. Neither friends nor family knew Chris had bought the drug and were shocked by his death. Word came back that nobody in his circle was taking it.

His parents also circulated warnings on social media. "I just don't know what to do to stop it happening to anybody else," said Lesley.

If DNP became a class-C drug, there would be penalties for buying and selling it, she reasoned.

The police have tried to track down the owner of the website who sold the pills to Chris. "We asked what happens if you ever find this person and the message was: 'We may be able to caution him,'" she said. "And you just think, sorry, our son has died and other people have died – caution? That's something you get if you nick a bar of chocolate, isn't it?"

Lesley is also keen for young people to understand that the drug can kill even if they have escaped harm before. Houston had been taking it for two years when she died.

Chris's mother has seen comments on websites from people who claim it is safe long-term. "Actually, it's not. And I feel that's a really important message to get out to people: to say, you might think you've got this under control, but actually it's poisoning you."

DNP causes the body's metabolism to speed up, burning fat but causing drastic overheating. Some who have died have been taken to hospital because of overheating. But there is little doctors can do.

At her inquest in April, Houston's family spoke of their horror that such drugs are available online. "To lose someone so young in this way only adds to our devastation," they said in a statement."There have been at least 62 further deaths related to DNP. We as a family are distraught and are keen to make sure no other family suffer in this way.

"Whilst the FSA has banned it from human consumption, its risks are not widely known and it does not seem to affect the ease at which it can be bought from the internet. It seems incomprehensible to us that such a toxic substance can be available in tablet form to be sold in the UK for human consumption across the internet."

Houston had told flatmates in Leeds she felt hot and unwell the night before she died, but insisted she did not need an ambulance. She was found dead the next day by one of her friends.

The cause of death was a mystery until tests were run on some capsules found in a brown envelope beside her bed. They were DNP, which she had bought online and had shipped from Spain. The inquest in Wakefield heard she had also been taking antidepressants.

Cheryl Gillan, MP for Chesham and Amersham, has held a meeting with the families and representatives of the police, FSA, MHRA and Department of Health, to try to unpick the regulatory complexities around DNP and work out how best to proceed. She has also been in touch with Google – which said it did not allow promotion of drugs making false or misleading health claims, citing anabolic steroids and weight-loss drugs – and with the Home Office.

She said she was still talking to the many agencies involved, and it appeared European regulation could also play a part. But in principle, she has a simple goal: "I'm looking at whether it will be helpful to have DNP brought under the Misuse of Drugs Act [as a class-C]. My aim would be to make sure that this drug is not marketed as a weight-loss drug."

Caroline Nokes, MP for Romsey and Southampton North, raised the issue in prime minister's questions in October, calling for urgent action to block the importation of dangerous chemicals sold in capsule form as diet drugs, which can be intended only for human consumption.

David Cameron responded by promising to look at "whether further legislative or regulatory action can be taken in order to protect people from substances that may be safe in other circumstances, but should not be marketed in this way".

DNP has a range of uses: as a pesticide, in explosives and in photographic developing. But, Nokes says, "in capsule form, it can't have any other function than for human consumption. I think it should be banned in capsule form. There needs to be real scrutiny of what is coming in through our borders."

Nokes also thinks more needs to be done to warn young people about buying it online. "Young people are dying from taking this stuff. The desperation to lose weight is encouraging them to take risks nobody should be taking. Because it is not licensed as a medicine doesn't mean it is not being widely regarded as one."

.