Two men in intensive care after taking part in a clinical trial remain critically ill and are unlikely to make any early improvement according to their doctor.
One of the two critically ill men has been named as Ryan Wilson, a 21-year-old student whose mother is from Dublin.
Ryan's mother Marion told the Daily Mirror: "He's in really bad shape."
Her estranged husband William is understood to have travelled from his home in Scotland to be with his son.
Six men were taken ill during the trial in London to test a drug designed to treat chronic inflammatory conditions and leukaemia.
Four are in serious condition but have shown signs of improvement.
"The two men remain critical and it could be a while until they show significant change," said Northwick Park Hospital's Clinical Director of Intensive Care, Ganesh Suntharalingham.
"The four who are seriously unwell are continuing to show signs of improvement but it is still early days," he added in a statement.
"We are still treating their inflammatory response following the drug trial ... and are providing full supportive care for the organs affected by this process."
Thomas Hanke, chief scientific officer of TeGenero AG
Relatives and friends of the two men have said the volunteers' features have become severely swollen. The hospital said patients in intensive care often needed a lot of fluid which could result in severe swelling and said this could reduce as they recover.
Suntharalingham said the six men could make a full recovery but added it was too early to make predictions.
He said he did now know what had caused the violent reaction. The trial was being run by U.S. drug research company Parexel International Corp. on behalf of German pharmaceutical company TeGenero AG.
They were testing a new biotech drug known as TGN 1412.
Parexel said they had operated within industry guidelines and TeGenero said they were shocked by the developments.
Thomas Hanke, chief scientific officer of the German firm, said in a statement the new medicine had shown no safety problems in previous laboratory trials.
Raste Khan, a volunteer who was given a plaebo in the drug trial
He said they were "shocked" by the developments and, asked by reporters if the company had apologised, he replied "yes."
One of the men given the placebo told Sky Newshow he had watched in horror as the six collapsed around him.
Raste Khan said the volunteers were writhing in agony, continually vomiting and screaming about the pain in their heads. He said they became ill minutes after taking the drug.
"This one man was yelling 'doctor, my head hurts, my back hurts. I need help, I can't breathe.' He was just shouting and rambling to himself," he said.
"It was like Russian Roulette - two of us got away and were lucky."
Britain's Medicines and Healthcare products Regulatory Agency (MHRA) have said they are working with police and the Department of Health to investigate what went wrong.
A spokesman said the inquiry would probably take weeks rather than days.
Volunteer Khan told Sky the men were paid just over £2,000 sterling (€2,900) to take part. The trial was in the first phase, when healthy humans test the drug.
As soon as the men fell ill, the MHRA suspended the trial and notified other European regulatory bodies about it.