Two men are critically ill and another four are seriously ill in the intensive care unit of Northwick Park hospital after taking an anti-inflammatory drug as part of a drug trial.
Myfanwy Marshall's boyfriend is one of those whose condition is critical. Doctors have warned he could die at any time.
She says he decided to take part in the trial - which paid about £2,000 - because he needed some money to pay his bills.
"He saw the ad, he told me it was for a leukaemia drug. He'd taken part in trials before and been fine.
"I didn't want him to do it, but he said he was helping mankind, helping scientific knowledge."
Myfanwy, 35, said he went into the Parexel unit at Northwick Park to begin the trial on Monday morning.
She received a call from the hospital at 3am the next day.
Doctors said her boyfriend had had a bad reaction to the drug.
She said the man she knew was "completely gorgeous, a beautiful person, really buoyant, oozing with charm and really muscly."
Now, she says, her 28-year-old boyfriend who runs a bar in London, has changed beyond all recognition.
'In the dark'
"I went in expecting to see his smiley face and curly black hair.
"But he was completely lifeless. He's like a shell of who he is.
"He can't even move his eyelids.
"This machine is pumping out his lungs. His chest is puffed out, his face is puffed out like the elephant man.
"A day ago I was talking to him and he was fine and now they are saying he could die at any moment."
She said his friends cannot bear to see him. His parents, who live abroad, are currently trying to get over to the UK.
Myfanwy added: "I can't hold his hand because of the tubes. But I have to stay there because I can see beyond the wires. I know he's in there.
"I sit and talk into his ear and tell his body to heal itself."
Myfanwy said doctors were working round the clock to treat the affected men, and were talking to scientists in the US and Germany who had worked on the development of the drug.
But she added: "The doctors say they are in the dark. They don't know the drug or what it can do.
"It's a drug they haven't tested on humans before, so they don't know what they're dealing with. All they can do is look at how his body has reacted."
"They say he just needs to get the drug out of his system.
"They say he needs a miracle."