Eloise Aimee Parry, a university student from Shrewsbury, died after taking eight tablets that were said to contain 2,4-dinitrophenol (DNP).
No details have been given about the product but West Mercia Police said they were working to determine the exact contents of the pill and appealing for information on where she might have bought them online.
In a statement, chief inspector Jennifer Mattinson said: "We are undoubtedly concerned over the origin and sale of these pills and are working with partner agencies to establish where they were bought from and how they were advertised.
"The coroner's report will establish the exact cause of Eloise's death but we urge the public to be incredibly careful when purchasing medicine or supplements over the internet. Substances from unregistered websites could put your health at risk as they could be extremely harmful, out-of-date or fake.”
The UK’s Food Standards Agency (FSA) said: "We advise the public not to take any tablets or powders containing DNP, as it is an industrial chemical and not fit for human consumption. It can be extremely dangerous to human health."
Several fatalities have bought the chemical – often used by bodybuilders to burn fat – into focus over the past few years. In the past the FSA, as well as other EU national authorities, have issued warnings on the substance and even threatened illicit online traders with prosecution.
According to a report published last June in the Emergency Medical Journal, the number of DNP poisoning cases had increased substantially in the last six years. Cases from toxic centre phone records increased from three in both 2007 and 2011 to 22 in 2013, while the number on online database TOXBASE rose from six in 2011 to 35 in 2012 right up to 331 in 2013.
In a statement to the public, Fiona Parry said her daughter had calmly driven herself to hospital after feeling unwell on 12th April.
“She had taken even more of these 'slimming tablets' than recommended on the pack and had no idea just how dangerous they really were. How many of us have ever thought 'If one tablet works, surely it won't hurt to take one or two more?'” she said.
Although lucid when she arrived at A&E, panic hit when the toxicity report came back.
“The drug was in her system, there was no antidote, two tablets was a lethal dose - and she had taken eight.”
Her metabolism soared and her temperature rose. Her mother said it was as if “she was literally burning up from within”.
Eloise Aimee Parry stopped breathing independently and later her heart stopped.