One of six men who fell seriously ill after taking part in a drug trial has told the Sun newspaper he felt like his brain was "on fire".
The men were all treated at Northwick Park Hospital
Student Nav Modi, 24, speaking for the first time, said the experience was a "terrible nightmare".
He said: "I felt my head swelling up like an elephant's -I thought my eyeballs were going to pop out."
Mr Modi and the other volunteers had an inflammatory response after being hired to test the drug TGN1412.
They had been given the drug, created by German pharmaceutical company TeGenero, by medical research company Parexel.
The previously healthy men had volunteered to test the medication, which was designed to treat rheumatoid arthritis, leukaemia and multiple sclerosis.
Mr Modi, of Forest Gate, East London, told The Sun he was gripped by pain just an hour after the drug was given to him and the other men.
"I was suddenly gripped by a pain I can barely begin to describe."
Despite screaming to a doctor, he said he was told to "lie down" and brought a paracetamol tablet - which he vomited back up within five minutes.
Mr Modi said: "Then somehow the pain got even worse with the pressure in my head so intense it was like a truck had been parked on it."
The pain then moved to his back.
He said: "I felt even worse than before and I was conscious of bucking and writhing in the bed as they tried to get an oxygen mask on me.
"It felt like a terrible nightmare."
He said he took part in the trial at London's Northwick Park Hospital to earn £2,000 to pay for a new laptop.
The latest statement from Northwick Park Hospital, issued on Wednesday, said two of the six men had been allowed home after responding "very well" to treatment.
One man is still in a critical condition while the other three continue to make a good recovery.