Two men remain critically ill and four others are in a serious condition after suffering a violent reaction while taking part in a clinical drugs trial.
The six are being treated at Northwick Park hospital
All are still in intensive care in Northwick Park Hospital, north-west London, after falling ill on Monday.
Myfanwy Marshall told BBC News her boyfriend's body was badly swollen and she had been told he could die.
One of the drugs companies involved in the trial said it has apologised to the families of the men.
But relatives are said to be unhappy with the information given from the firm behind the anti-inflammatory drug.
The families had been given "mixed messages" during two meetings with pharmaceutical company TeGenero AG, which manufactures the drug and Parexel, which ran the trial, it was claimed.
Lawyer Ann Alexander, representing one of the critically ill men, told the BBC the companies had been asked whether any of the animals used to test the drug had died.
"I understand that yesterday, they were told a dog had died during the testing. Today that was denied," she said.
But TeGenero has described the reactions as "shocking developments" and said the new medicine showed no signs of problems in earlier tests.
Ms Alexander added: "These are ordinary health people who have been involved in a clinical trial and unfortunately everything seems to have gone wrong.
"I think the most scary thing for the families..is that they don't know what the outcome is going to be".
As there was no antidote and doctors did not know what had happened, they were having to treat the patients on a "symptomatic basis", she said.
It was the first time the drug TBN1412, designed to treat conditions such as rheumatoid arthritis and leukaemia, had been tested on humans.
Within hours of taking it on Monday, the six young volunteers had to be admitted to intensive care.
Ms Marshall, 35, whose boyfriend is critically ill, said the normally healthy 28-year-old's face was so puffed, he "looks like the Elephant Man".
She said he was completely lifeless, unable even to move an eyelid.
"They just keep saying he's very, very sick and we are doing all we can," she added.
The hospital says it is continuing to treat the men's "inflammatory disorder" and would do all it could to support the families.
TeGenero's chief scientific officer Thomas Hanke said the company's first concern was making sure the patients got the best treatment possible and to support the familes.
He added: "They were shocked, devastated. We deeply understand that they are. We are devastated at these shocking developments which we were not anticipating."
He said TeGenero AG had apologised to the men's families.
An investigation by inspectors from the Medicines and Healthcare products Regulatory Agency (MHRA) at the private research unit on the hospital's grounds has begun.
American company Parexel, which runs the unit, said it followed recommended guidelines in its trial.
Professor Kent Woods, from the MHRA, said they would be looking at whether the reaction was caused by a manufacturing problem, contamination, a dosing error or whether it was some "completely unanticipated side effect of the drug in humans".
The Medical Research Council said that while it was "an unfortunate and extremely rare event", such clinical trials were essential for the development of new and better treatments.
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