David Cameron has pledged to "look carefully" at how to highlight the dangers of banned weight-loss drugs being sold legally for other uses.
He spoke during Prime Minister's Questions, in the Commons, in the wake of an inquest into the death of Leeds medical student Sarah Houston.
The 23-year-old died after taking the toxic slimming pill DNP.
Although banned for human consumption, DNP is easily available online as a legitimate pesticide.
Mr Cameron said: "This isn't an easy issue because of course the substance is banned as a slimming drug but, as I understand it, it is legal as a herbicide.
"And so we have to look carefully across government about what more we can do to warn people about these things."
Ms Houston, a bulimia sufferer, from Chesham, Buckinghamshire, was found dead in her bedroom in Leeds in September last year after taking the drug at the same time as antidepressants.
Her family have called for the weight-loss drug to be made illegal and say it is "incomprehensible" it can be bought online.