One of the men given a dummy drug as part of the clinical trial that left six men seriously ill has said the study was like "Russian roulette".
Raste Khan said that the test ward in north-west London became a "living hell" as the men spasmed in agony.
Two remain critically ill but four have shown signs of improvement.
A solicitor representing one man said it was not clear if successful animal tests had been previously held.
Ann Alexander, whose 29-year-old client is on a life support machine, said: "There is confusion about whether the drug had actually been tested successfully and safely on animals before the tests on these volunteers."
She said the "problem" needed to be "investigated urgently".
"The other six have obviously gone through a terrible time. I don't know what their families are going through."
Mr Khan from Barry, near Cardiff, was given a placebo, and spoke to the BBC courtesy of the Sun newspaper.
He said: "It was Russian roulette. There were eight of us. Two of us were really lucky."
He added: "They must be going through a terrible time at the moment. I really hope they're going to get through this.
"All we can do is hope they're going to be fine."
He said that luckily he felt OK after being given his injection but that the others on the trial began falling "like dominos".
"The gentleman on my left said, 'I've got really bad headache pains.' He said he was hot and then he started hyper-ventilating.
"Then they tried to calm him down and then he passed out and came back to consciousness, he vomited and then they got a big bin liner from somewhere for him to vomit in," he said.
Then he described how another volunteer became unwell and said he could not control himself and needed the toilet.
"Everything was unplugged from him, he stood up, took several steps and he fainted. He was quite a big guy, it took quite a few nurses to help him up."
Earlier, Mr Khan had told the Sun newspaper: "Some screamed out that their heads felt like they were going to explode."
Clinical Director of Intensive care Ganesh Suntharalingham at Northwick Park Hospital said: "Of the six patients admitted to critical care, the four who are seriously unwell are continuing to show signs of improvement but it is still early days.
"The other two men remain critical and it could be a while until they show significant change."
One of the critically ill men has been named as student Ryan Wilson, 21, of Highbury, north London.
Another who was taken seriously ill has been confirmed as a New Zealander. The New Zealand High Commission said he was "conscious and has spoken to hospital staff".
'No signs of problems'
It was the first time the drug TGN1412, designed to treat conditions such as rheumatoid arthritis and leukaemia, had been tested on humans.
American company Parexel, which ran the trial, said it had followed recommended guidelines.
TeGenero, which manufactures the anti-inflammatory drug, said it apologised to the sick men's families and said the medicine had showed no signs of problems in earlier tests.
Chief scientific officer Thomas Hanke added the company's first concern now was making sure the patients got the best treatment possible and to support the families
Scotland Yard said officers are talking to the Medicines and Healthcare products Regulatory Agency and doctors.
The MHRA is 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".